fmt-merge-msg: don't use static buffer in record_person

fmt-merge-msg: don't use static buffer in record_person

The record_person function just parses out the "name" field
of the person line in a commit and adds it to a string_list.
The only reason we need an extra buffer is that the
string_list functions require a NUL-terminated string.

Instead of the static buffer, we can just allocate a
temporary NUL-terminated copy. In addition to removing a
useless limit, this removes the only user of MAX_GITNAME
outside of ident.c.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>