cli: git fetch: allow short alias for --all-remotes

[calebdw]

Allows using -a as a short alias for --all-remotes in the git fetch command.

Thanks!

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[yuja]

I think -a/--all in this context is confusing because it could potentially mean "all branches"/"bookmarks" (if we had some config knob for the default fetch spec.)

[calebdw]

Even git uses --all to fetch from all remotes....I'm not sure that I would think of fetch as fetching all bookmarks/branches?

Granted, git uses -a for --append:

    -a, --[no-]append     append to .git/FETCH_HEAD instead of overwriting