Added repo cherry-pick command

It is undesired to have the same Change-Id:-line for two separate
commits, and when cherry-picking, the user must manually change it.

If this is not done, bad things may happen (such as when the user
is uploading the cherry-picked commit to Gerrit, it will instead
see it as a new patch-set for the original change, or worse).

repo cherry-pick works the same was as git cherry-pick, except that
it replaces the Change-Id with a new one and adds a reference
back to the commit from where it was picked.

On failures (when git can not successfully apply the cherry-picked
commit), instructions will be written to the user.

Change-Id: I5a38b89839f91848fad43386d43cae2f6cdabf83

subcmds/cherry_pick.py

@@ -0,0 +1,114 @@
+#
+# Copyright (C) 2010 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import sys, re, string, random, os
+from command import Command
+from git_command import GitCommand
+
+CHANGE_ID_RE = re.compile(r'^\s*Change-Id: I([0-9a-f]{40})\s*$')
+
+class CherryPick(Command):
+  common = True
+  helpSummary = "Cherry-pick a change."
+  helpUsage = """
+%prog <sha1>
+"""
+  helpDescription = """
+'%prog' cherry-picks a change from one branch to another.
+The change id will be updated, and a reference to the old
+change id will be added.
+"""
+
+  def _Options(self, p):
+    pass
+
+  def Execute(self, opt, args):
+    if len(args) != 1:
+      self.Usage()
+
+    reference = args[0]
+
+    p = GitCommand(None,
+                   ['rev-parse', '--verify', reference],
+                   capture_stdout = True,
+                   capture_stderr = True)
+    if p.Wait() != 0:
+      print >>sys.stderr, p.stderr
+      sys.exit(1)
+    sha1 = p.stdout.strip()
+
+    p = GitCommand(None, ['cat-file', 'commit', sha1], capture_stdout=True)
+    if p.Wait() != 0:
+      print >>sys.stderr, "error: Failed to retrieve old commit message"
+      sys.exit(1)
+    old_msg = self._StripHeader(p.stdout)
+
+    p = GitCommand(None,
+                   ['cherry-pick', sha1],
+                   capture_stdout = True,
+                   capture_stderr = True)
+    status = p.Wait()
+
+    print >>sys.stdout, p.stdout
+    print >>sys.stderr, p.stderr
+
+    if status == 0:
+      # The cherry-pick was applied correctly. We just need to edit the
+      # commit message.
+      new_msg = self._Reformat(old_msg, sha1)
+
+      p = GitCommand(None, ['commit', '--amend', '-F', '-'],
+                     provide_stdin = True,
+                     capture_stdout = True,
+                     capture_stderr = True)
+      p.stdin.write(new_msg)
+      if p.Wait() != 0:
+        print >>sys.stderr, "error: Failed to update commit message"
+        sys.exit(1)
+
+    else:
+      print >>sys.stderr, """\
+NOTE: When committing (please see above) and editing the commit message,
+please remove the old Change-Id-line and add:
+"""
+      print >>sys.stderr, self._GetReference(sha1)
+      print >>sys.stderr
+
+  def _IsChangeId(self, line):
+    return CHANGE_ID_RE.match(line)
+
+  def _GetReference(self, sha1):
+    return "(cherry picked from commit %s)" % sha1
+
+  def _StripHeader(self, commit_msg):
+    lines = commit_msg.splitlines()
+    return "\n".join(lines[lines.index("")+1:])
+
+  def _Reformat(self, old_msg, sha1):
+    new_msg = []
+
+    for line in old_msg.splitlines():
+      if not self._IsChangeId(line):
+        new_msg.append(line)
+
+    # Add a blank line between the message and the change id/reference
+    try:
+      if new_msg[-1].strip() != "":
+        new_msg.append("")
+    except IndexError:
+      pass
+
+    new_msg.append(self._GetReference(sha1))
+    return "\n".join(new_msg)