init: add --manifest-depth for shallow manifest clone

People rarely care about the history of the manifest repo.  Add a
parameter to specify depth for the manifest.

For now, make the default behavior the same as the current behavior.  At
a future date, the default will be changed to 1.  People who need the
full history should begin passing --manifest-depth=0 to preserve the
behavior when the default changes.

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, https://crbug.com/gerrit/16358
Change-Id: I9130fed3eaed656435c778a85cfe9d04e3a4a6a0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/349814
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Xin Li <delphij@google.com>

repo

@@ -316,6 +316,10 @@ 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=0, metavar='DEPTH',
+                   help='create a shallow clone of the manifest repo with '
+                        'given depth (0 for full clone); see git clone '
+                        '(default: %default)')
 
   # Options that only affect manifest project, and not any of the projects
   # specified in the manifest itself.