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>