Optionally preserve line endings

CheckForWindowsLineEndings is a Chromium presubmit to make sure that
\r\n line endings (Windows line endings) don't get committed. Among
other things these line endings mess up the license checks, leading to
cryptic errors.

The problem is that ChangedContents() was stripping line endings, making
any checking for them futile. This change adds an option to preserve
line endings, to be used by Chromium's CheckForWindowsLineEndings.

It's not clear how long this has been broken.

See also crrev.com/c/2911914 which must land after this.

Bug: 801033
Change-Id: I7cdb9b3581788624f9eca081da43bb070ee412a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2910488
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>

presubmit_support.py

@@ -1008,7 +1008,7 @@ class AffectedFile(object):
         pass  # File not found?  That's fine; maybe it was deleted.
     return self._cached_new_contents[:]
 
-  def ChangedContents(self):
+  def ChangedContents(self, keeplinebreaks=False):
     """Returns a list of tuples (line number, line text) of all new lines.
 
      This relies on the scm diff output describing each changed code section
@@ -1016,20 +1016,27 @@ class AffectedFile(object):
 
      ^@@ <old line num>,<old size> <new line num>,<new size> @@$
     """
-    if self._cached_changed_contents is not None:
+    # Don't return cached results when line breaks are requested.
+    if not keeplinebreaks and self._cached_changed_contents is not None:
       return self._cached_changed_contents[:]
-    self._cached_changed_contents = []
+    result = []
     line_num = 0
 
-    for line in self.GenerateScmDiff().splitlines():
+    # The keeplinebreaks parameter to splitlines must be True or else the
+    # CheckForWindowsLineEndings presubmit will be a NOP.
+    for line in self.GenerateScmDiff().splitlines(keeplinebreaks):
       m = re.match(r'^@@ [0-9\,\+\-]+ \+([0-9]+)\,[0-9]+ @@', line)
       if m:
         line_num = int(m.groups(1)[0])
         continue
       if line.startswith('+') and not line.startswith('++'):
-        self._cached_changed_contents.append((line_num, line[1:]))
+        result.append((line_num, line[1:]))
       if not line.startswith('-'):
         line_num += 1
+    # Don't cache results with line breaks.
+    if keeplinebreaks:
+      return result;
+    self._cached_changed_contents = result
     return self._cached_changed_contents[:]
 
   def __str__(self):