Handle case where 'git remote prune' needs to be run

Handle the case when this error occurs:
    error: some local refs could not be updated; try running
     'git remote prune origin' to remove any old, conflicting branches

This is usually caused by a reference getting changed from a file to a
directory.

For example:
  Initially someone creates a branch 'foo' and it is stored as:
    .git/refs/remotes/origin/foo

  Then later on it is decided to change the layout structure where 'foo'
  is a directory with branches below it:
    .git/refs/remotes/origin/foo/master

  The problem occurs when someone still has
  '.git/refs/remotes/origin/foo' on their system and does a repo sync.
  When this occurs the error message for needing to do a
  'git remote prune origin' occurs.

Now when doing a 'git fetch' if the error message from git says that a
'git remote prune' is needed, it will do the prune and then retry the
fetch.

Change-Id: I4c6f5aa6bd932f0ef7a39134400bedd52e82f633
Signed-off-by: John L. Villalovos <john.l.villalovos@intel.com>

project.py

@@ -1871,10 +1871,22 @@ class Project(object):
 
     ok = False
     for _i in range(2):
-      ret = GitCommand(self, cmd, bare=True, ssh_proxy=ssh_proxy).Wait()
+      gitcmd = GitCommand(self, cmd, bare=True, capture_stderr=True,
+                          ssh_proxy=ssh_proxy)
+      ret = gitcmd.Wait()
       if ret == 0:
         ok = True
         break
+      # If needed, run the 'git remote prune' the first time through the loop
+      elif (not _i and
+            "error:" in gitcmd.stderr and
+            "git remote prune" in gitcmd.stderr):
+        prunecmd = GitCommand(self, ['remote', 'prune', name], bare=True,
+                              capture_stderr=True, ssh_proxy=ssh_proxy)
+        if prunecmd.Wait():
+          print(prunecmd.stderr, file=sys.stderr)
+          break
+        continue
       elif current_branch_only and is_sha1 and ret == 128:
         # Exit code 128 means "couldn't find the ref you asked for"; if we're in sha1
         # mode, we just tried sync'ing from the upstream field; it doesn't exist, thus