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>

subcmds/init.py

@@ -109,6 +109,10 @@ to update the working directory files.
     Args:
       opt: options from optparse.
     """
+    # Normally this value is set when instantiating the project, but the
+    # manifest project is special and is created when instantiating the
+    # manifest which happens before we parse options.
+    self.manifest.manifestProject.clone_depth = opt.manifest_depth
     if not self.manifest.manifestProject.Sync(
         manifest_url=opt.manifest_url,
         manifest_branch=opt.manifest_branch,