Preserve merges when doing a rebase

When a person is using git-review after having merged in a branch, the
default rebase behavior as currently implemented will rebase all of the
commits that are inside of the merge. This is almost never what someone
wants to do.

Add -p to the rebase commands which will tell git to preserve the merges
and only rebase the merge commit itself.

Explicitly preserve original SHA1 from before the rebase, since git may
not set ORIG_HEAD as expected depending the operations performed.

Change-Id: I8a5f0307113cd0cf0d08c664c13a0676400bd05c

git_review/cmd.py

@@ -56,6 +56,7 @@ DEFAULTS = dict(hostname=False, port='29418', project=False,
 
 _branch_name = None
 _has_color = None
+_orig_head = None
 
 
 class colors:
@@ -456,15 +457,28 @@ def check_remote(branch, remote, hostname, port, project):
 
 def rebase_changes(branch, remote, interactive=True):
 
+    global _orig_head
+
     remote_branch = "remotes/%s/%s" % (remote, branch)
 
     if not update_remote(remote):
         return False
 
+    # since the value of ORIG_HEAD may not be set by rebase as expected
+    # for use in undo_rebase, make sure to save it explicitly
+    cmd = "git rev-parse HEAD"
+    (status, output) = run_command_status(cmd)
+    if status != 0:
+        print("Errors running %s" % cmd)
+        if interactive:
+            print(output)
+        return False
+    _orig_head = output
+
     if interactive:
-        cmd = "git rebase -i %s" % remote_branch
+        cmd = "git rebase -p -i %s" % remote_branch
     else:
-        cmd = "git rebase %s" % remote_branch
+        cmd = "git rebase -p %s" % remote_branch
 
     (status, output) = run_command_status(cmd, GIT_EDITOR='true')
     if status != 0:
@@ -476,7 +490,11 @@ def rebase_changes(branch, remote, interactive=True):
 
 
 def undo_rebase():
-    cmd = "git reset --hard ORIG_HEAD"
+    global _orig_head
+    if not _orig_head:
+        return True
+
+    cmd = "git reset --hard %s" % _orig_head
     (status, output) = run_command_status(cmd)
     if status != 0:
         print("Errors running %s" % cmd)

git_review/tests/test_git_review.py

@@ -100,7 +100,7 @@ class GitReviewTestCase(tests.BaseGitReviewTestCase):
                             'create conflict with master')
 
         exc = self.assertRaises(Exception, self._run_git_review)
-        self.assertIn("Errors running git rebase -i remotes/gerrit/master",
+        self.assertIn("Errors running git rebase -p -i remotes/gerrit/master",
                       exc.args[0])
 
     def test_upload_without_rebase(self):
@@ -115,7 +115,7 @@ class GitReviewTestCase(tests.BaseGitReviewTestCase):
                             self._dir('test', 'new_test_file.txt'))
 
         review_res = self._run_git_review('-v')
-        self.assertIn("Running: git rebase -i remotes/gerrit/master",
+        self.assertIn("Running: git rebase -p -i remotes/gerrit/master",
                       review_res)
         self.assertEqual(self._run_git('rev-parse', 'HEAD^1'), head_1)