Add support for partial clone.

A new option, --partial-clone is added to 'repo init' which tells repo
to utilize git's partial clone functionality, which reduces disk and
bandwidth usage when downloading by omitting blob downloads initially.
Different from restricting clone-depth, the user will have full access
to change history, etc., as the objects are downloaded on demand.

Change-Id: I60326744875eac16521a007bd7d5481112a98749
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/229532
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Xin Li <delphij@google.com>