check-commit: check capitalization in summary lines

At the moment, check-commit will complain about the topic being capitalized,
but not the summary that comes after it.  This diff corrects that deficiency.

contrib/check-commit

@@ -25,6 +25,7 b' errors = ['
     (r"^# .*\n(?!merge with )[^#]\S+[^:] ",
      "summary line doesn't start with 'topic: '"),
     (r"^# .*\n[A-Z][a-z]\S+", "don't capitalize summary lines"),
+    (r"^# .*\n[^\n]*: *[A-Z][a-z]\S+", "don't capitalize summary lines"),
     (r"^# .*\n.*\.\s+$", "don't add trailing period on summary line"),
     (r"^# .*\n.{78,}", "summary line too long"),
     (r"^\+\n \n", "adds double empty line"),