rebase: Merge --ff-only if the PR is a merge

If the HEAD of the PR is a merge commit (has more than 1 parent), ask
the user if they would like to do a `merge --ff-only` instead, as
normally rebasing a merge commit results in horrors and nightmares.

This feature should be considered experimental, in the future this might
be done by default without asking or an option might be added to force
this behaviour beforehand to avoid annoying interactive questions.

Fixes sociomantic-tsunami#183.

git-hub

@@ -1851,11 +1851,24 @@ class RebaseCmd (PullUtil):
         starting = args.action is None
         try:
             if starting:
-                a = []
-                if config.forcerebase:
-                    a.append('--force')
-                a.append('FETCH_HEAD')
-                git_quiet(1, 'rebase', *a)
+                parents = git('show --quiet --format="%P" FETCH_HEAD').split()
+                # Last commit is a merge commit, so ask the user to merge
+                # --ff-only instead
+                if len(parents) > 1:
+                    answer = ask("The last commit in the pull request is a "
+                        "merge commit, do a merge --ff-only instead of a "
+                        "rebase?", default="yes")
+                    if answer == 'no':
+                        sys.exit(0)
+                    assert answer == 'yes'
+                    git_quiet(1, 'merge --ff-only FETCH_HEAD')
+                    return
+                else:
+                    a = []
+                    if config.forcerebase:
+                        a.append('--force')
+                    a.append('FETCH_HEAD')
+                    git_quiet(1, 'rebase', *a)
             else:
                 git('rebase', args.action)
         except subprocess.CalledProcessError as e: