Improve checkout performance for the common unmodified case

Most projects will have their branch heads matching in all branches,
so switching between them should be just a matter of updating the
work tree's HEAD symref.  This can be done in pure Python, saving
quite a bit of time over forking 'git checkout'.

Signed-off-by: Shawn O. Pearce <sop@google.com>

subcmds/start.py

@@ -49,7 +49,8 @@ revision specified in the manifest.
     pm.end()
 
     if err:
-      err.sort()
       for p in err:
-        print >>sys.stderr, "error: cannot start in %s" % p.relpath
+        print >>sys.stderr,\
+          "error: %s/: cannot start %s" \
+          % (p.relpath, nb)
       sys.exit(1)