Include mutually exclusive flags in help text

[joshfrench]

What type of PR is this?

• feature

What this PR does / why we need it:

The default help text did not include mutually exclusive flags; now it does:

cmd := &cli.Command{
        MutuallyExclusiveFlags: []cli.MutuallyExclusiveFlags{{
            Flags: [][]cli.Flag{
                {
                    &cli.StringFlag{
                        Name: "foo",
                    },
                },
                {
                    &cli.BoolFlag{
                        Name:     "bar",
                        Category: "Category",
                    },
                },
                {
                    &cli.BoolFlag{
                        Name:     "baz",
                        Category: "Category",
                    },
                },
            },
        }},
    }

GLOBAL OPTIONS:
   --foo value
   --help, -h   show help (default: false)

   Category

   --bar  (default: false)
   --baz  (default: false)

Changes:

• SetCategory method added to Flag interface.
• Category field and propagateCategory method added to MutuallyExclusiveFlags, allowing mutex group to set a category and propagate it downward to all flags within.
• Mutex group category downward-propagation added to command and subcommand setup, prior to computing visible categories. This implies that any category set on a mutex flag itself will be ignored if no group category is set, or overridden if one is.
• Replaced cmd.Flags with cmd.allFlags() when calculating visible flags and visible flag categories, to ensure downward-propagated group categories are included in help text.

Which issue(s) this PR fixes:

Fixes #1858

Special notes for your reviewer:

I considered two approaches, described in #1858 . This is the simpler of the two, where no attempt is made to display or categorize mutually exclusive flags in any special way. Under this approach the changes are minimal and ~users don't need to consider any special cases when using mutually exclusive flags.~ Flags within a mutex group now ignore their own Category and inherit it from the group instead, but there's precedent with the mutex group's Required field so it's not an entirely new concept.

Testing

Unit tests are updated to reflect both uncategorized and categorized output.

Release Notes

Mutually exclusive flags are now included in the default help text.

[joshfrench]

@dearchap I'm stalled at the following test error:

# 🚨 ---> Hey! <---
# The unified diff above indicates that the public API surface area
# has changed. If you feel that the changes are acceptable for the
# v3.x series, please run the following command to promote the
# current go docs:
#
#     make v3approve

Should I run that and get the tests passing, or would you like to review the changes first? Thanks!

[dearchap]

@joshfrench Yes the changes look good for the most part. Can you run make v3approve and push ?

[joshfrench]

@dearchap pushed!