Extend Go completions and revamp zsh comp (#1070) (#1070)

Replace the current Zsh completion with a Zsh completion solution based
on Go completions. This allows to support custom completions (based
on Go completions), but also to standardize the behavior of completion
across all shells.

Also, add support to Go completions for the bash completion annotations:
  BashCompFilenameExt (including Command.MarkFlagFilename() family)
                       - still supported by zsh
  BashCompSubdirsInDir - now supported by zsh
  BashCompOneRequiredFlag (including Command.MarkFlagRequired() family)
                       - now supported by zsh and fish

Finally, remove the suggestin of the = form of flag completion.
The = form is supported, but it will not be suggested to avoid having
duplicated suggestions.

zsh_completions.md

@@ -1,39 +1,47 @@
-## Generating Zsh Completion for your cobra.Command
+## Generating Zsh Completion For Your cobra.Command
 
-Cobra supports native Zsh completion generated from the root `cobra.Command`.
-The generated completion script should be put somewhere in your `$fpath` named
-`_<YOUR COMMAND>`.
+Please refer to [Shell Completions](shell_completions.md) for details.
 
-### What's Supported
+## Zsh completions standardization
 
-* Completion for all non-hidden subcommands using their `.Short` description.
-* Completion for all non-hidden flags using the following rules:
-  * Filename completion works by marking the flag with `cmd.MarkFlagFilename...`
-    family of commands.
-  * The requirement for argument to the flag is decided by the `.NoOptDefVal`
-    flag value - if it's empty then completion will expect an argument.
-  * Flags of one of the various `*Array` and `*Slice` types supports multiple
-    specifications (with or without argument depending on the specific type).
-* Completion of positional arguments using the following rules:
-  * Argument position for all options below starts at `1`. If argument position
-    `0` is requested it will raise an error.
-  * Use `command.MarkZshCompPositionalArgumentFile` to complete filenames. Glob
-    patterns (e.g. `"*.log"`) are optional - if not specified it will offer to
-    complete all file types.
-  * Use `command.MarkZshCompPositionalArgumentWords` to offer specific words for
-    completion. At least one word is required.
-  * It's possible to specify completion for some arguments and leave some
-    unspecified (e.g. offer words for second argument but nothing for first
-    argument). This will cause no completion for first argument but words
-    completion for second argument.
-  * If no argument completion was specified for 1st argument (but optionally was
-    specified for 2nd) and the command has `ValidArgs` it will be used as
-    completion options for 1st argument.
-  * Argument completions only offered for commands with no subcommands.
+Cobra 1.1 standardized its zsh completion support to align it with its other shell completions.  Although the API was kept backwards-compatible, some small changes in behavior were introduced.
 
-### What's not yet Supported
+### Deprecation summary
 
-* Custom completion scripts are not supported yet (We should probably create zsh
-  specific one, doesn't make sense to re-use the bash one as the functions will
-  be different).
-* Whatever other feature you're looking for and doesn't exist :)
+See further below for more details on these deprecations.
+
+* `cmd.MarkZshCompPositionalArgumentFile(pos, []string{})` is no longer needed.  It is therefore **deprecated** and silently ignored.
+* `cmd.MarkZshCompPositionalArgumentFile(pos, glob[])` is **deprecated** and silently ignored.
+  * Instead use `ValidArgsFunction` with `ShellCompDirectiveFilterFileExt`.
+* `cmd.MarkZshCompPositionalArgumentWords()` is **deprecated** and silently ignored.
+  * Instead use `ValidArgsFunction`.
+
+### Behavioral changes
+
+**Noun completion**
+|Old behavior|New behavior|
+|---|---|
+|No file completion by default (opposite of bash)|File completion by default; use `ValidArgsFunction` with `ShellCompDirectiveNoFileComp` to turn off file completion on a per-argument basis|
+`cmd.MarkZshCompPositionalArgumentFile(pos, []string{})` used to turn on file completion on a per-argument position basis|File completion for all arguments by default; `cmd.MarkZshCompPositionalArgumentFile()` is **deprecated** and silently ignored|
+|`cmd.MarkZshCompPositionalArgumentFile(pos, glob[])` used to turn on file completion **with glob filtering** on a per-argument position basis (zsh-specific)|`cmd.MarkZshCompPositionalArgumentFile()` is **deprecated** and silently ignored; use `ValidArgsFunction` with `ShellCompDirectiveFilterFileExt` for file **extension** filtering (not full glob filtering)|
+|`cmd.MarkZshCompPositionalArgumentWords(pos, words[])` used to provide completion choices on a per-argument position basis (zsh-specific)|`cmd.MarkZshCompPositionalArgumentWords()` is **deprecated** and silently ignored; use `ValidArgsFunction` to achieve the same behavior|
+
+**Flag-value completion**
+
+|Old behavior|New behavior|
+|---|---|
+|No file completion by default (opposite of bash)|File completion by default; use `RegisterFlagCompletionFunc()` with `ShellCompDirectiveNoFileComp` to turn off file completion|
+|`cmd.MarkFlagFilename(flag, []string{})` and similar used to turn on file completion|File completion by default; `cmd.MarkFlagFilename(flag, []string{})` no longer needed in this context and silently ignored|
+|`cmd.MarkFlagFilename(flag, glob[])`  used to turn on file completion **with glob filtering** (syntax of `[]string{"*.yaml", "*.yml"}` incompatible with bash)|Will continue to work, however, support for bash syntax is added and should be used instead so as to work for all shells (`[]string{"yaml", "yml"}`)|
+|`cmd.MarkFlagDirname(flag)` only completes directories (zsh-specific)|Has been added for all shells|
+|Completion of a flag name does not repeat, unless flag is of type `*Array` or `*Slice` (not supported by bash)|Retained for `zsh` and added to `fish`|
+|Completion of a flag name does not provide the `=` form (unlike bash)|Retained for `zsh` and added to `fish`|
+
+**Improvements**
+
+* Custom completion support (`ValidArgsFunction` and `RegisterFlagCompletionFunc()`)
+* File completion by default if no other completions found
+* Handling of required flags
+* File extension filtering no longer mutually exclusive with bash usage
+* Completion of directory names *within* another directory
+* Support for `=` form of flags