Avoid unnecessary invocations of "git fetch" (reland)

If we already have the correct commit, it is not necessary to fetch.  Any
fetches can takes minutes due to gerrit problems, and often fail
completely, so it is helpful to avoid them completely whenever we can.

Reland now that skia autoroller does its own fetch.

Bug: 938627

Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1504102
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

Change-Id: I5a6877125c682ff0d98bd6d2ffe495245f74c513
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1512024
Auto-Submit: Michael Spang <spang@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

gclient_scm.py

@@ -657,14 +657,18 @@ class GitWrapper(SCMWrapper):
         raise gclient_utils.Error('Invalid Upstream: %s' % upstream_branch)
 
     self._SetFetchConfig(options)
-    self._Fetch(options, prune=options.force)
 
+    # Fetch upstream if we don't already have |revision|.
     if not scm.GIT.IsValidRevision(self.checkout_path, revision, sha_only=True):
-      # Update the remotes first so we have all the refs.
-      remote_output = scm.GIT.Capture(['remote'] + verbose + ['update'],
-              cwd=self.checkout_path)
-      if verbose:
-        self.Print(remote_output)
+      self._Fetch(options, prune=options.force)
+
+      if not scm.GIT.IsValidRevision(self.checkout_path, revision,
+                                     sha_only=True):
+        # Update the remotes first so we have all the refs.
+        remote_output = scm.GIT.Capture(['remote'] + verbose + ['update'],
+                cwd=self.checkout_path)
+        if verbose:
+          self.Print(remote_output)
 
     revision = self._AutoFetchRef(options, revision)