Sync: Add option to prune refs during sync

By passing --prune to the sync command, the --prune option is
given to the `git fetch`, causing refs that no longer exist on
the remote to be removed.

Change-Id: I3cedacce14276d96ac2d5aabf2d07fd05e92bc02

project.py

@@ -1110,7 +1110,8 @@ class Project(object):
       clone_bundle=True,
       no_tags=False,
       archive=False,
-      optimized_fetch=False):
+      optimized_fetch=False,
+      prune=False):
     """Perform only the network IO portion of the sync process.
        Local working directory/branch state is not affected.
     """
@@ -1181,7 +1182,7 @@ class Project(object):
     if (need_to_fetch
         and not self._RemoteFetch(initial=is_new, quiet=quiet, alt_dir=alt_dir,
                                   current_branch_only=current_branch_only,
-                                  no_tags=no_tags)):
+                                  no_tags=no_tags, prune=prune)):
       return False
 
     if self.worktree:
@@ -1795,7 +1796,8 @@ class Project(object):
                    initial=False,
                    quiet=False,
                    alt_dir=None,
-                   no_tags=False):
+                   no_tags=False,
+                   prune=False):
 
     is_sha1 = False
     tag_name = None
@@ -1908,6 +1910,9 @@ class Project(object):
     else:
       cmd.append('--tags')
 
+    if prune:
+      cmd.append('--prune')
+
     spec = []
     if not current_branch_only:
       # Fetch whole repo