SPARK-1431: Allow merging conflicting pull requests

Sometimes if there is a small conflict it's nice to be able to just
manually fix it up rather than have another RTT with the contributor.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #342 from pwendell/merge-conflicts and squashes the following commits:

cdce61a [Patrick Wendell] SPARK-1431: Allow merging conflicting pull requests

dev/merge_spark_pr.py

@@ -87,11 +87,20 @@ def merge_pr(pr_num, target_ref):
   run_cmd("git fetch %s %s:%s" % (PUSH_REMOTE_NAME, target_ref, target_branch_name))
   run_cmd("git checkout %s" % target_branch_name)
 
-  run_cmd(['git', 'merge', pr_branch_name, '--squash'])
+  had_conflicts = False
+  try:
+    run_cmd(['git', 'merge', pr_branch_name, '--squash'])
+  except Exception as e:
+    msg = "Error merging: %s\nWould you like to manually fix-up this merge?" % e
+    continue_maybe(msg)
+    msg = "Okay, please fix any conflicts and 'git add' conflicting files... Finished?"
+    continue_maybe(msg)
+    had_conflicts = True
 
   commit_authors = run_cmd(['git', 'log', 'HEAD..%s' % pr_branch_name, 
     '--pretty=format:%an <%ae>']).split("\n")
-  distinct_authors = sorted(set(commit_authors), key=lambda x: commit_authors.count(x), reverse=True)
+  distinct_authors = sorted(set(commit_authors), key=lambda x: commit_authors.count(x), 
+    reverse=True)
   primary_author = distinct_authors[0]
   commits = run_cmd(['git', 'log', 'HEAD..%s' % pr_branch_name, 
     '--pretty=format:%h [%an] %s']).split("\n\n")
@@ -105,6 +114,13 @@ def merge_pr(pr_num, target_ref):
 
   merge_message_flags += ["-m", authors]
 
+  if had_conflicts:
+    committer_name = run_cmd("git config --get user.name").strip()
+    committer_email = run_cmd("git config --get user.email").strip()
+    message = "This patch had conflicts when merged, resolved by\nCommitter: %s <%s>" % (
+      committer_name, committer_email)
+    merge_message_flags += ["-m", message]
+
   # The string "Closes #%s" string is required for GitHub to correctly close the PR
   merge_message_flags += ["-m",
     "Closes #%s from %s and squashes the following commits:" % (pr_num, pr_repo_desc)]
@@ -186,8 +202,10 @@ def maybe_cherry_pick(pr_num, merge_hash, default_branch):
   maybe_cherry_pick(pr_num, merge_hash, latest_branch)
   sys.exit(0)
 
-if bool(pr["mergeable"]) == False:
-  fail("Pull request %s is not mergeable in its current form" % pr_num)
+if not bool(pr["mergeable"]):
+  msg = "Pull request %s is not mergeable in its current form.\n" % pr_num + \
+    "Continue? (experts only!)"
+  continue_maybe(msg)
 
 print ("\n=== Pull Request #%s ===" % pr_num)
 print("title\t%s\nsource\t%s\ntarget\t%s\nurl\t%s" % (