Revert "Avoid unnecessary invocations of "git fetch""

This reverts commit 61a1b5f3.

Reason for revert:
It's blocking skia autoroller
https://skia-review.googlesource.com/c/buildbot/+/198700

Original change's description:
> Avoid unnecessary invocations of "git fetch"
> 
> 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.
> 
> Bug: 938627
> 
> Change-Id: I54494cad9b9aa0090f51674b6b9f56bb3e842f12
> 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>

TBR=spang@chromium.org,agable@chromium.org,ehmaldonado@chromium.org

Change-Id: If8bd26d74bf84ff0ca69378dbfe6b9369b9988d3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 938627
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1508980Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

gclient_scm.py

@@ -657,18 +657,14 @@ 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):
-      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)
+      # 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)