repo: rework parser setup to handle `init -c`

We added support for `repo init -c` to main.py, but not to the
launcher, so the -c option only works after the first init has
run which kind of defeats its purpose.  Rework the parser setup
so that we can tell it whether it's for "init" or "gitc-init"
and then add the -c option in the same way we do in main.py.

This has the benefit of getting the parser entirely out of the
module scope which makes it a lot easier to reason about, and
it means we can write some unittests.

Change-Id: Icbc2ec3aceb938d5a8f941d5fbce1548553dc5f7
Test: repo help init
Test: repo help gitc-init
Test: repo init -u https://android.googlesource.com/platform/manifest -c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255113
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>