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>
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