Print diff-index when tree is dirty

git_cl.py

Index: git_cl.py
diff --git a/git_cl.py b/git_cl.py
index 62f339d9a3e3e2534aa4801b72f5ccf7d635da15..fd426f05e7d171393e105e95d8492742862a2de7 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -999,10 +999,13 @@ def CMDpresubmit(parser, args):
 
   # Make sure index is up-to-date before running diff-index.
   RunGit(['update-index', '--refresh', '-q'], error_ok=True)
-  if not options.force and RunGit(['diff-index', 'HEAD']):
+  diff_index = RunGit(['diff-index', 'HEAD'])
+  if not options.force and diff_index:
     # TODO(maruel): Is this really necessary?
-    print ('Cannot presubmit with a dirty tree.\n'
-          'You must commit locally first (or use --force).')
+    print >> sys.stderr, (
+        'Cannot presubmit with a dirty tree.\n'
+        'You must commit locally first (or use --force).\n'
+        'git diff-index HEAD\n%s' % diff_index[:4096])
     return 1
 
   cl = Changelist()
@@ -1201,8 +1204,11 @@ def CMDupload(parser, args):
 
   # Make sure index is up-to-date before running diff-index.
   RunGit(['update-index', '--refresh', '-q'], error_ok=True)
-  if RunGit(['diff-index', 'HEAD']):
-    print 'Cannot upload with a dirty tree.  You must commit locally first.'
+  diff_index = RunGit(['diff-index', 'HEAD'])
+  if diff_index:
+    print >> sys.stderr, (
+        'Cannot upload with a dirty tree.  You must commit locally first.'
+        'git diff-index HEAD\n%s' % diff_index[:4096])
     return 1
 
   cl = Changelist()
@@ -1274,8 +1280,11 @@ def SendUpstream(parser, args, cmd):
 
   # Make sure index is up-to-date before running diff-index.
   RunGit(['update-index', '--refresh', '-q'], error_ok=True)
-  if RunGit(['diff-index', 'HEAD']):
-    print 'Cannot %s with a dirty tree.  You must commit locally first.' % cmd
+  diff_index = RunGit(['diff-index', 'HEAD'])
+  if diff_index:
+    print >> sys.stderr, (
+        'Cannot %s with a dirty tree.  You must commit locally first.'
+        'git diff-index HEAD\n%s' % (cmd, diff_index[:4096]))
     return 1
 
   # This rev-list syntax means "show all commits not in my branch that