pre-upload: skip merge commits

We don't have any checks that apply to merge commits, so skip them.

BUG=None
TEST=`repo upload` with merge commits skips them

Change-Id: I27441d1b052a6930c458b02fa754bc543b5f4c89
Reviewed-on: https://chromium-review.googlesource.com/1802576
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Allen Webb <allenwebb@google.com>
diff --git a/pre-upload.py b/pre-upload.py
index bb73ee0..f2938d5 100755
--- a/pre-upload.py
+++ b/pre-upload.py
@@ -401,7 +401,8 @@
 
 def _get_commits():
   """Returns a list of commits for this review."""
-  cmd = ['git', 'log', '%s..' % _get_upstream_branch(), '--format=%H']
+  cmd = ['git', 'log', '--no-merges', '--format=%H',
+         '%s..' % _get_upstream_branch()]
   return _run_command(cmd).split()