init: restore default --manifest-name

The merge of the repo & init parser missed this default.

When running `repo init ...` in an existing checkout but w/out the -m
option, then repo would error out complaining that -m is required when
it didn't do this before.

Change-Id: I58035d48cc413b5d373702b9dc3b9ecd3fd1e900
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/303945


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>