Skip changes that are NOPS when there are no diffs

CheckAddedDepsHaveTargetApprovals and CheckStableMojomChanges only look
at lines that have changed. This makes them expensive (OldContents() is
one of the most expensive presubmit functions) and guarantees that they
will not find anything when run with --all or --files, when there are no
changed lines.

Therefore this change pragmatically returns early from these functions
when no_diffs is true. This reduces "presubmit --all" times on my test
machine by about 6 minutes, or 5-10%, allowing for faster iteration
times.

Bug: 1309977
Change-Id: I3a947efa6a59d562aa27f80fdda3f7ad38993352
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3671444
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Jesse McKenna <jessemckenna@google.com>
Cr-Commit-Position: refs/heads/main@{#1010839}

PRESUBMIT.py

@@ -2237,7 +2237,7 @@ def CheckAddedDepsHaveTargetApprovals(input_api, output_api):
     # We rely on Gerrit's code-owners to check approvals.
     # input_api.gerrit is always set for Chromium, but other projects
     # might not use Gerrit.
-    if not input_api.gerrit:
+    if not input_api.gerrit or input_api.no_diffs:
         return []
     if (input_api.change.issue and input_api.gerrit.IsOwnersOverrideApproved(
             input_api.change.issue)):
@@ -5631,7 +5631,7 @@ def CheckStableMojomChanges(input_api, output_api):
         include_deletes=True,
         file_filter=lambda f: f.LocalPath().endswith(('.mojom')))
 
-    if not changed_mojoms:
+    if not changed_mojoms or input_api.no_diffs:
         return []
 
     delta = []

PRESUBMIT_test_mocks.py

@@ -79,6 +79,7 @@ class MockInputApi(object):
     self.change = MockChange([])
     self.presubmit_local_path = os.path.dirname(__file__)
     self.is_windows = sys.platform == 'win32'
+    self.no_diffs = False
 
   def CreateMockFileInPath(self, f_list):
     self.os_path.exists = lambda x: x in f_list