gclient_scm: Make calls to "git merge-base" non-fatal.

It might be the case for some repositories that there is no merge base
between the current git HEAD and origin/master, which causes git
merge-base to exit with a non-zero code and cause calls to gclient
status/diff/pack to fail, as well as gclient sync if the repository in
question has been removed from DEPS.

This is true for the external/webrtc/trunk/talk repository, for example.
Its recent release branches (branch-heads/45 all the way to /53 at
least) have no ancestry shared with its master branch, so gclient
sync'ing from a Chromium M51 checkout to an M52 one (where it's no
longer in DEPS) fails because of the failed git merge-base calls.

We now ignore failures and just don't specify a merge base when calling
"git diff".

BUG=633962
R=iannucci@chromium.org,agable@chromium.org,maruel@chromium.org

Review-Url: https://codereview.chromium.org/2215673002

gclient_scm.py

@@ -292,8 +292,11 @@ class GitWrapper(SCMWrapper):
     """
 
   def diff(self, options, _args, _file_list):
-    merge_base = self._Capture(['merge-base', 'HEAD', self.remote])
-    self._Run(['diff', merge_base], options)
+    try:
+      merge_base = [self._Capture(['merge-base', 'HEAD', self.remote])]
+    except subprocess2.CalledProcessError:
+      merge_base = []
+    self._Run(['diff'] + merge_base, options)
 
   def pack(self, _options, _args, _file_list):
     """Generates a patch file which can be applied to the root of the
@@ -302,9 +305,12 @@ class GitWrapper(SCMWrapper):
     The patch file is generated from a diff of the merge base of HEAD and
     its upstream branch.
     """
-    merge_base = self._Capture(['merge-base', 'HEAD', self.remote])
+    try:
+      merge_base = [self._Capture(['merge-base', 'HEAD', self.remote])]
+    except subprocess2.CalledProcessError:
+      merge_base = []
     gclient_utils.CheckCallAndFilter(
-        ['git', 'diff', merge_base],
+        ['git', 'diff'] + merge_base,
         cwd=self.checkout_path,
         filter_fn=GitDiffFilterer(self.relpath, print_func=self.Print).Filter)
 
@@ -785,11 +791,14 @@ class GitWrapper(SCMWrapper):
       self.Print('________ couldn\'t run status in %s:\n'
                  'The directory does not exist.' % self.checkout_path)
     else:
-      merge_base = self._Capture(['merge-base', 'HEAD', self.remote])
-      self._Run(['diff', '--name-status', merge_base], options,
+      try:
+        merge_base = [self._Capture(['merge-base', 'HEAD', self.remote])]
+      except subprocess2.CalledProcessError:
+        merge_base = []
+      self._Run(['diff', '--name-status'] + merge_base, options,
                 stdout=self.out_fh)
       if file_list is not None:
-        files = self._Capture(['diff', '--name-only', merge_base]).split()
+        files = self._Capture(['diff', '--name-only'] + merge_base).split()
         file_list.extend([os.path.join(self.checkout_path, f) for f in files])
 
   def GetUsableRev(self, rev, options):