t7502: narrow checks for author/committer name in template

t7502: narrow checks for author/committer name in template

t7502.20 and t7502.21 check that the author and committer
name are mentioned in the commit message template under
certain circumstances. However, they end up checking a much
larger and unnecessary portion of the template. Let's narrow
their checks to the specific lines.

While we're at it, let's give these tests more descriptive
names, so their purposes are more obvious.

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