In Git.unstaged_files, handle the case when there are no changes.

This is a follow-up fix to a mistake in http://crrev.com/2590693002.

BUG=662584

Review-Url: https://codereview.chromium.org/2605933002
Cr-Commit-Position: refs/heads/master@{#440887}

third_party/WebKit/Tools/Scripts/webkitpy/common/checkout/scm/git.py

@@ -121,9 +121,12 @@ class Git(SCM):
         """
         # `git status -z` is a version of `git status -s`, that's recommended
         # for machine parsing. Lines are terminated with NUL rather than LF.
+        change_lines = self._run_git(['status', '-z']).rstrip('\x00')
+        if not change_lines:
+            return {}  # No changes.
         unstaged_changes = {}
-        change_lines = self._run_git(['status', '-z']).rstrip('\x00').split('\x00')
-        for line in change_lines:
+        for line in change_lines.split('\x00'):
+            assert len(line) > 4, 'Unexpected change line format %s' % line
             if line[1] == ' ':
                 continue  # Already staged for commit.
             path = line[3:]

third_party/WebKit/Tools/Scripts/webkitpy/common/checkout/scm/scm_unittest.py

@@ -255,7 +255,7 @@ class GitTestWithMock(SCMTestBase):
 
     def test_unstaged_files(self):
         scm = self.make_scm()
-        status_lines = [
+        lines = [
             ' M d/modified.txt',
             ' D d/deleted.txt',
             '?? d/untracked.txt',
@@ -263,8 +263,7 @@ class GitTestWithMock(SCMTestBase):
             'M  d/modified-staged.txt',
             'A  d/added-staged.txt',
         ]
-        # pylint: disable=protected-access
-        scm._run_git = lambda args: '\x00'.join(status_lines) + '\x00'
+        scm._run_git = lambda _: '\x00'.join(lines) + '\x00'  # pylint: disable=protected-access
         self.assertEqual(
             scm.unstaged_changes(),
             {
@@ -272,3 +271,8 @@ class GitTestWithMock(SCMTestBase):
                 'd/deleted.txt': 'D',
                 'd/untracked.txt': '?',
             })
+
+    def test_unstaged_files_with_no_changes(self):
+        scm = self.make_scm()
+        scm._run_git = lambda _: '\x00'  # pylint: disable=protected-access
+        self.assertEqual(scm.unstaged_changes(), {})