t7500: rename commit tests script to comply with naming convention

t7500: rename commit tests script to comply with naming convention

When the test naming convention was documented[1] the commit script
was not renamed.

Update the test description to note that the tests fall into four
general categories: template, sign-off, -F and squash tests.

Chose to not add "File" to the new script name as that did not seem to
convey the current test contents for that switch.

[1] f50c9f76c ("Rename some test scripts and describe the naming convention", 2005-05-15)

Signed-off-by: Stephen P. Smith <ischis2@cox.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>