Remove rev-parse --verify FETCH_HEAD

It's not used and causes some problems if refs on clone aren't set
properly.

R=ehmaldonado@chromium.org

Bug: 1049610
Change-Id: I71c1215c9a3355dd54e2d66860d755c6fe8032db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2224853
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

subprocess2.py

@@ -200,6 +200,8 @@ def call(args, **kwargs):
   Automatically convert stdout=PIPE or stderr=PIPE to VOID.
   In no case they can be returned since no code path raises
   subprocess2.CalledProcessError.
+
+  Returns exit code.
   """
   if kwargs.get('stdout') == PIPE:
     kwargs['stdout'] = VOID