Improve "dcommit in git repo" error message.

git_cl.py

 #!/usr/bin/env python
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Copyright (C) 2008 Evan Martin <martine@danga.com>
 
 """A git-command for integrating reviews on Rietveld."""
 
 from distutils.version import LooseVersion
@@ skipping 26 matching lines @@
 from third_party import colorama
 from third_party import httplib2
 from third_party import upload
 import auth
 import breakpad  # pylint: disable=W0611
 import clang_format
 import dart_format
 import fix_encoding
 import gclient_utils
 import git_common
+from git_footers import get_footer_svn_id
 import owners
 import owners_finder
 import presubmit_support
 import rietveld
 import scm
 import subcommand
 import subprocess2
 import watchlists
 
 __version__ = '1.0'
@@ skipping 2581 matching lines @@
 
 def IsFatalPushFailure(push_stdout):
   """True if retrying push won't help."""
   return '(prohibited by Gerrit)' in push_stdout
 
 
 @subcommand.usage('[upstream branch to apply against]')
 def CMDdcommit(parser, args):
   """Commits the current changelist via git-svn."""
   if not settings.GetIsGitSvn():
-    message = """This doesn't appear to be an SVN repository.
-If your project has a git mirror with an upstream SVN master, you probably need
-to run 'git svn init', see your project's git mirror documentation.
-If your project has a true writeable upstream repository, you probably want
-to run 'git cl land' instead.
-Choose wisely, if you get this wrong, your commit might appear to succeed but
-will instead be silently ignored."""
+    if get_footer_svn_id():
+      # If it looks like previous commits were mirrored with git-svn.
+      message = """This repository appears to be a git-svn mirror, but no
+upstream SVN master is set. You probably need to run 'git auto-svn' once."""
+    else:
+      message = """This doesn't appear to be an SVN repository.
+If your project has a true, writeable git repository, you probably want to run
+'git cl land' instead.
+If your project has a git mirror of an upstream SVN master, you probably need
+to run 'git svn init'.
+
+Using the wrong command might cause your commit to appear to succeed, and the
+review to be closed, without actually landing upstream. If you choose to
+proceed, please verify that the commit lands upstream as expected."""
     print(message)
     ask_for_data('[Press enter to dcommit or ctrl-C to quit]')
   return SendUpstream(parser, args, 'dcommit')
 
 
 @subcommand.usage('[upstream branch to apply against]')
 def CMDland(parser, args):
   """Commits the current changelist via git."""
-  if settings.GetIsGitSvn():
+  if settings.GetIsGitSvn() or get_footer_svn_id():
     print('This appears to be an SVN repository.')
     print('Are you sure you didn\'t mean \'git cl dcommit\'?')
+    print('(Ignore if this is the first commit after migrating from svn->git)')
     ask_for_data('[Press enter to push or ctrl-C to quit]')
   return SendUpstream(parser, args, 'land')
 
 
 @subcommand.usage('<patch url or issue id>')
 def CMDpatch(parser, args):
   """Patches in a code review."""
   parser.add_option('-b', dest='newbranch',
                     help='create a new branch off trunk for the patch')
   parser.add_option('-f', '--force', action='store_true',
@@ skipping 698 matching lines @@
 if __name__ == '__main__':
   # These affect sys.stdout so do it outside of main() to simplify mocks in
   # unit testing.
   fix_encoding.fix_encoding()
   colorama.init()
   try:
     sys.exit(main(sys.argv[1:]))
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(1)