sync: Enable use of git clone --reference

Use git clone to initialize a new repository, and when possible
allow callers to use --reference to reuse an existing checkout as
the initial object storage area for the new checkout.

Change-Id: Ie27f760247f311ce484c6d3e85a90d94da2febfc
Signed-off-by: Shawn O. Pearce <sop@google.com>
3 files changed