Telemetry perf trybots: diff against master

git cl upload uses the tracking branch to diff against unless otherwise
specified. This change specifies the origin/master branch should be
used.

BUG=401180

Review URL: https://codereview.chromium.org/487433005

Cr-Commit-Position: refs/heads/master@{#291689}

tools/telemetry/telemetry/core/backends/remote/trybot_browser_finder.py

@@ -133,7 +133,7 @@ class PossibleTrybotBrowser(possible_browser.PossibleBrowser):
     # Upload the CL to rietveld and run a try job.
     returncode, out, err = self._RunProcess([
         'git', 'cl', 'upload', '-f', '--bypass-hooks', '-m',
-        'CL for perf tryjob'
+        'CL for perf tryjob', 'origin/master'
     ])
     if returncode:
       logging.error('Could upload to reitveld, error %s', err)

tools/telemetry/telemetry/core/backends/remote/trybot_browser_finder_unittest.py

@@ -167,7 +167,7 @@ class TrybotBrowserFinderTest(unittest.TestCase):
         (['git', 'checkout', '-b', 'telemetry-tryjob'], (0, None, None)),
         (['git', 'commit', '-a', '-m', 'bisect config'], (0, None, None)),
         (['git', 'cl', 'upload', '-f', '--bypass-hooks', '-m',
-         'CL for perf tryjob'],
+         'CL for perf tryjob', 'origin/master'],
          (0, 'stuff https://codereview.chromium.org/12345 stuff', None)),
         (['git', 'cl', 'try', '-m', 'tryserver.chromium.perf', '-b', bot],
          (0, None, None)),