Changed "gclient sync -D" logic to compare against the expected DEPS version

Also add a better warning if 'gclient sync -D' fails due to a modified file.
I've tested this code via deleting a DEP and running gclient sync -D.

Bug: 981149
Change-Id: I97035ac238d163ccb1684c3ee423c223ed0f6299
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1891830
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Anthony Polito <apolito@google.com>

gclient.py

@@ -1687,6 +1687,13 @@ it or fix the checkout.
                   should_recurse=False,
                   relative=None,
                   condition=None))
+          if modified_files and self._options.delete_unversioned_trees:
+            print('\nWARNING: \'%s\' is no longer part of this client.\n'
+                  'Despite running \'gclient sync -D\' no action was taken '
+                  'as there are modifications.\nIt is recommended you revert '
+                  'all changes or run \'gclient sync -D --force\' next '
+                  'time.' % entry_fixed)
+          else:
             print('\nWARNING: \'%s\' is no longer part of this client.\n'
                   'It is recommended that you manually remove it or use '
                   '\'gclient sync -D\' next time.' % entry_fixed)

gclient_scm.py

@@ -912,10 +912,11 @@ class GitWrapper(SCMWrapper):
       self.Print('________ couldn\'t run status in %s:\n'
                  'The directory does not exist.' % self.checkout_path)
     else:
-      try:
-        merge_base = [self._Capture(['merge-base', 'HEAD', self.remote])]
-      except subprocess2.CalledProcessError:
       merge_base = []
+      if self.url:
+        _, base_rev = gclient_utils.SplitUrlRevision(self.url)
+        if base_rev:
+          merge_base = [base_rev]
       self._Run(
           ['-c', 'core.quotePath=false', 'diff', '--name-status'] + merge_base,
           options, always_show_header=options.verbose)

tests/gclient_scm_test.py

@@ -307,7 +307,8 @@ class ManagedGitWrapperTestCase(BaseGitWrapperTestCase):
     file_path = join(self.base_path, 'a')
     with open(file_path, 'a') as f:
       f.writelines('touched\n')
-    scm = gclient_scm.GitWrapper(self.url, self.root_dir,
+    scm = gclient_scm.GitWrapper(
+        self.url + '@069c602044c5388d2d15c3f875b057c852003458', self.root_dir,
         self.relpath)
     file_list = []
     scm.status(options, self.args, file_list)
@@ -317,6 +318,22 @@ class ManagedGitWrapperTestCase(BaseGitWrapperTestCase):
          '069c602044c5388d2d15c3f875b057c852003458\' in \'%s\'\n\nM\ta\n') %
             join(self.root_dir, '.'))
 
+
+  def testStatusNewNoBaseRev(self):
+    if not self.enabled:
+      return
+    options = self.Options()
+    file_path = join(self.base_path, 'a')
+    with open(file_path, 'a') as f:
+      f.writelines('touched\n')
+    scm = gclient_scm.GitWrapper(self.url, self.root_dir, self.relpath)
+    file_list = []
+    scm.status(options, self.args, file_list)
+    self.assertEqual(file_list, [file_path])
+    self.checkstdout(
+        ('\n________ running \'git -c core.quotePath=false diff --name-status'
+         '\' in \'%s\'\n\nM\ta\n') % join(self.root_dir, '.'))
+
   def testStatus2New(self):
     if not self.enabled:
       return
@@ -327,7 +344,8 @@ class ManagedGitWrapperTestCase(BaseGitWrapperTestCase):
       with open(file_path, 'a') as f:
         f.writelines('touched\n')
       expected_file_list.extend([file_path])
-    scm = gclient_scm.GitWrapper(self.url, self.root_dir,
+    scm = gclient_scm.GitWrapper(
+        self.url + '@069c602044c5388d2d15c3f875b057c852003458', self.root_dir,
         self.relpath)
     file_list = []
     scm.status(options, self.args, file_list)