presubmit: support 'Bug: 1234, 5678'

The current BugsFromDescription code returns ['1234, 5678'] but I think it
should return ['1234', '5678']. For 'BUG=1234, 5678' this already works.

This was preventing me from uploading
https://chromium-review.googlesource.com/c/catapult/+/1481239
because of https://cs.chromium.org/chromium/src/third_party/catapult/PRESUBMIT.py?q=catapult/PRESUBMIT.py&sq=package:chromium&g=0&l=52

See https://logs.chromium.org/logs/catapult/buildbucket/cr-buildbucket.appspot.com/8920885706080647120/+/steps/presubmit/0/stdout

Change-Id: Ibb3fca8c7a6ec664cdf9a38a8a7ac2bf66cb73ed
Reviewed-on: https://chromium-review.googlesource.com/c/1481240
Auto-Submit: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>

presubmit_support.py

@@ -1032,7 +1032,11 @@ class Change(object):
   def BugsFromDescription(self):
     """Returns all bugs referenced in the commit description."""
     tags = [b.strip() for b in self.tags.get('BUG', '').split(',') if b.strip()]
-    footers = git_footers.parse_footers(self._full_description).get('Bug', [])
+    footers = []
+    unsplit_footers = git_footers.parse_footers(self._full_description).get(
+        'Bug', [])
+    for unsplit_footer in unsplit_footers:
+      footers += [b.strip() for b in unsplit_footer.split(',')]
     return sorted(set(tags + footers))
 
   def ReviewersFromDescription(self):

tests/presubmit_unittest.py

@@ -1553,12 +1553,19 @@ class ChangeUnittest(PresubmitTestsBase):
     self.assertEquals('WHIZ=bang\nbar\nFOO=baz', change.FullDescriptionText())
     self.assertEquals({'WHIZ': 'bang', 'FOO': 'baz'}, change.tags)
 
-  def testBugsFromDescription(self):
+  def testBugsFromDescription_MixedTagsAndFooters(self):
     change = presubmit.Change(
-        '', 'foo\nBUG=2,1\n\nChange-Id: asdf\nBug: 3',
+        '', 'foo\nBUG=2,1\n\nChange-Id: asdf\nBug: 3, 6',
         self.fake_root_dir, [], 0, 0, '')
-    self.assertEquals(['1', '2', '3'], change.BugsFromDescription())
-    self.assertEquals('1,2,3', change.BUG)
+    self.assertEquals(['1', '2', '3', '6'], change.BugsFromDescription())
+    self.assertEquals('1,2,3,6', change.BUG)
+
+  def testBugsFromDescription_MultipleFooters(self):
+    change = presubmit.Change(
+        '', 'foo\n\nChange-Id: asdf\nBug: 1\nBug:4,  6',
+        self.fake_root_dir, [], 0, 0, '')
+    self.assertEquals(['1', '4', '6'], change.BugsFromDescription())
+    self.assertEquals('1,4,6', change.BUG)
 
   def testReviewersFromDescription(self):
     change = presubmit.Change(