sync: Correct imports of `R_HEADS` and `HEAD`

`R_HEADS` is imported twice, from both the git_refs and project
modules.

It is actually defined in git_refs, and in project it is imported
from there, so the import of `R_HEADS` from project in the sync
module is redundant.  Remove it.

`HEAD` is imported from project, but like `R_HEADS` it is actually
defined in git_refs.  Import it from git_refs instead.

Change-Id: I8e2b0217d0d9f9f4ee5ef5b8cd0b026174ac52f4

subcmds/sync.py

@@ -37,13 +37,11 @@ except ImportError:
     return (256, 256)
 
 from git_command import GIT
-from git_refs import R_HEADS
-from project import HEAD
+from git_refs import R_HEADS, HEAD
 from project import Project
 from project import RemoteSpec
 from command import Command, MirrorSafeCommand
 from error import RepoChangedException, GitError
-from project import R_HEADS
 from project import SyncBuffer
 from progress import Progress