Revert "Revert "Add git coommit completions"" (#869)

Reverts #868

I found that this works, if you source these files like
```
use aliases/git/git-aliases.nu *
use custom-completions/git/git-completions.nu *
```
but if you source them like this, it doesn't work.
```
use custom-completions/git/git-completions.nu *
use aliases/git/git-aliases.nu *
```

This changed recently somewhere in nushell.

custom-completions/git/git-completions.nu

@@ -428,8 +428,46 @@ export extern "git diff" [
 export extern "git commit" [
  --all(-a) # automatically stage all modified and deleted files
  --amend # amend the previous commit rather than adding a new one
- --message(-m)  # specify the commit message rather than opening an editor
+ --message(-m): string # specify the commit message rather than opening an editor
  --no-edit # don't edit the commit message (useful with --amend)
+ --reuse-message(-C): string # reuse the message from a previous commit
+ --reedit-message(-c): string # reuse and edit message from a commit
+ --fixup: string # create a fixup/amend commit
+ --squash: string # squash commit for autosquash rebase
+ --reset-author # reset author information
+ --short # short-format output for dry-run
+ --branch # show branch info in short-format
+ --porcelain # porcelain-ready format for dry-run
+ --long # long-format output for dry-run
+ --null(-z) # use NUL instead of LF in output
+ --file(-F): string # read commit message from file
+ --author: string # override commit author
+ --date: string # override author date
+ --template(-t): string # use commit message template file
+ --signoff(-s) # add Signed-off-by trailer
+ --no-signoff # do not add Signed-off-by trailer
+ --trailer: string # add trailer to commit message
+ --no-verify(-n) # bypass pre-commit and commit-msg hooks
+ --verify # do not bypass pre-commit and commit-msg hooks
+ --allow-empty # allow commit with no changes
+ --allow-empty-message # allow commit with empty message
+ --cleanup: string # cleanup commit message
+ --edit(-e) # edit commit message
+ --no-edit # do not edit commit message
+ --amend # amend previous commit
+ --include(-i) # include given paths in commit
+ --only(-o) # commit only specified paths
+ --pathspec-from-file: string # read pathspec from file
+ --pathspec-file-nul # use NUL character for pathspec file
+ --untracked-files(-u): string # show untracked files
+ --verbose(-v) # show diff in commit message template
+ --quiet(-q) # suppress commit summary
+ --dry-run # show paths to be committed without committing
+ --status # include git-status output in commit message
+ --no-status # do not include git-status output
+ --gpg-sign(-S):string # GPG-sign commit
+ --no-gpg-sign # do not GPG-sign commit
+ ...pathspec: string # commit files matching pathspec
 ]
 
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