Bug 1333564 - [manifestparser] Stop supporting ';' as a valid comment character, r=jmaher

It turns out there are shockingly few cases of manifestparser manifests that actually use the ';'
character as a comment. Because we will soon allow inline comments, deprecating the use of ';' will
ensure that values are allowed to have semicolons in them.

Even without inline comments, might as well enforce consistency across manifests.

MozReview-Commit-ID: AEPPQFdNXG0

python/mozbuild/mozbuild/backend/recursivemake.py

@@ -1433,7 +1433,7 @@ class RecursiveMakeBackend(CommonBackend):
     def _write_master_test_manifest(self, path, manifests):
         with self._write_file(path) as master:
             master.write(
-                '; THIS FILE WAS AUTOMATICALLY GENERATED. DO NOT MODIFY BY HAND.\n\n')
+                '# THIS FILE WAS AUTOMATICALLY GENERATED. DO NOT MODIFY BY HAND.\n\n')
 
             for manifest in sorted(manifests):
                 master.write('[include:%s]\n' % manifest)

python/mozbuild/mozbuild/test/backend/test_recursivemake.py

@@ -519,7 +519,7 @@ class TestRecursiveMakeBackend(BackendTester):
 
         lines = [l.strip() for l in open(x_master, 'rt').readlines()]
         self.assertEqual(lines, [
-            '; THIS FILE WAS AUTOMATICALLY GENERATED. DO NOT MODIFY BY HAND.',
+            '# THIS FILE WAS AUTOMATICALLY GENERATED. DO NOT MODIFY BY HAND.',
             '',
             '[include:dir1/xpcshell.ini]',
             '[include:xpcshell.ini]',