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>

subcmds/init.py

@@ -41,6 +41,13 @@ The optional -m argument can be used to specify an alternate manifest
 to be used. If no manifest is specified, the manifest default.xml
 will be used.
 
+The --reference option can be used to point to a directory that
+has the content of a --mirror sync. This will make the working
+directory use as much data as possible from the local reference
+directory when fetching from the server. This will make the sync
+go a lot faster by reducing data traffic on the network.
+
+
 Switching Manifest Branches
 ---------------------------
 
@@ -71,7 +78,9 @@ to update the working directory files.
     g.add_option('--mirror',
                  dest='mirror', action='store_true',
                  help='mirror the forrest')
-
+    g.add_option('--reference',
+                 dest='reference',
+                 help='location of mirror directory', metavar='DIR')
 
     # Tool
     g = p.add_option_group('repo Version options')
@@ -115,6 +124,9 @@ to update the working directory files.
       r.ResetFetch()
       r.Save()
 
+    if opt.reference:
+      m.config.SetString('repo.reference', opt.reference)
+
     if opt.mirror:
       if is_new:
         m.config.SetString('repo.mirror', 'true')