init: change --depth default to 1 for manifest repo

People rarely care about the history of the manifest repo.  Change
the default to 1 to speed up initial setup.  If people really want
the full history, they can pass --manifest-depth=0.

We can't reuse the existing --depth option because that applies to
all projects we clone, not just the manifest repo.

Bug: https://crbug.com/gerrit/16193
Change-Id: Ideab1712e9ffc743b9bae97903e074d7d429b134
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/343435
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: LaMont Jones <lamontjones@google.com>

repo

@@ -316,6 +316,9 @@ def InitParser(parser, gitc_init=False):
                    help='download the manifest as a static file '
                         'rather then create a git checkout of '
                         'the manifest repo')
+  group.add_option('--manifest-depth', type='int', default=1, metavar='DEPTH',
+                   help='create a shallow clone of the manifest repo with '
+                        'given depth; see git clone (default: %default)')
 
   # Options that only affect manifest project, and not any of the projects
   # specified in the manifest itself.