check-commit: disallow capitalization only right after topic

It should be okay to write "topic: change 'default: True' to 'default:
on'".

Differential Revision: https://phab.mercurial-scm.org/D5429

contrib/check-commit

@@ -34,7 +34,7 b' errors = ['
     (commitheader + r"(?!merge with )[^#]\S+[^:] ",
      "summary line doesn't start with 'topic: '"),
     (afterheader + r"[A-Z][a-z]\S+", "don't capitalize summary lines"),
-    (afterheader + r"[^\n]*: *[A-Z][a-z]\S+", "don't capitalize summary lines"),
+    (afterheader + r"^\S+: *[A-Z][a-z]\S+", "don't capitalize summary lines"),
     (afterheader + r"\S*[^A-Za-z0-9-_]\S*: ",
      "summary keyword should be most user-relevant one-word command or topic"),
     (afterheader + r".*\.\s*\n", "don't add trailing period on summary line"),