bash-completion: refactor unique subcommand processing
bash-completion: refactor unique subcommand processing

Let's keep the main processing loop free of subcommand implementations
by pulling the existing help & start commands into dedicated functions.
Having a single giant function is harder to track as we add more and
more logic in.

Bug: https://crbug.com/gerrit/14797
Change-Id: I2b62dc430c0e7574f09aa4838f4ef03fbe4bf7fb
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/312903
Reviewed-by: Xin Li <93465216fd46a440eb04b27d6e009aced8b0c153@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@google.com>

GerritCodeReview/git-repo,couchbasedeps/git-repo,GerritCodeReview/git-repo,couchbasedeps/git-repo,lewixliu/git-repo,lewixliu/git-repo