post-receive-email: optional message line count limit

post-receive-email: optional message line count limit

We have become used to the features of svnmailer when used with Subversion,
and one of those useful features is that it can limit the maximum length
(in lines) of a commit email message. This is terribly useful since once the
goes beyond a reasonable number of lines, nobody is going to read the remainder,
and if they really want the entire contents of the commits, they can use
git itself to get them using the revision IDs present in the message already.

Change the post-receive-email script to respond to an 'emailmaxlines' config key
which, if specified, will limit the number of lines generated (including
headers); any lines beyond the limit are suppressed, and a final line is added
indicating the number that were suppressed.

Signed-off-by: Kevin P. Fleming <kpfleming@digium.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>