messages: capitalization and punctuation exceptions

messages: capitalization and punctuation exceptions

These are conscious violations of the usual rules for error messages,
based on this reasoning:

 - If an error message is directly followed by another sentence, it
   needs to be properly terminated with a period, lest the grammar
   looks broken and becomes hard to read.

 - That second sentence isn't actually an error message any more, so
   it should abide to conventional language rules for good looks and
   legibility. Arguably, these should be converted to advice
   messages (which the user can squelch, too), but that's a much
   bigger effort to get right.

 - Neither of these apply to the first hunk in do_exec(), but this
   two-line message looks just too much like a real sentence to not
   terminate it. Also, leaving it alone would make it asymmetrical
   to the other hunk.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>