hooks: add --fix option to auto-apply hook fixes

Pass the --fix flag as a keyword argument "fix" to the hook main
function. This allows hooks (such as git-repohooks) to decouple
automated fix application from the -y/--yes flag so that -y can
answer yes to upload confirmation prompts without triggering file
mutations.

Companion change in git-repohooks:
https://gerrit-review.googlesource.com/c/git-repohooks/+/621761

Bug: 546510319
Test: python3 -m pytest tests/test_hooks.py

Change-Id: If0288d4791fc0a2aba6e88854e3aa81b0923b664
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/619281
Commit-Queue: Rahul Yadav <yadavrah@google.com>
Tested-by: Rahul Yadav <yadavrah@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

completion.zsh

@@ -400,6 +400,7 @@ _repo() {
         '--no-verify[Do not verify]' \
         '--verify[Verify]' \
         '--ignore-hooks[Ignore hooks]' \
+        '--fix[Automatically fix]' \
         '*: :->project'
       ;;
     version)