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>

t/t7500-commit-template-squash-signoff.sh

@@ -5,7 +5,7 @@
 
 test_description='git commit
 
-Tests for selected commit options.'
+Tests for template, signoff, squash and -F functions.'
 
 . ./test-lib.sh