t7502: rename commit test script to comply with naming convention

t7502: rename commit test script to comply with naming convention

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

The test description for t7502 indicates that the test file is to
contain porcelain type options for the commit command.

The tests don't fall into a single category.  There are tests for
cleanup, sign-off, multiple message options, etc.

Rename the t7502-commit.sh to t7502-commit-porcelain.sh which reflects
the high level nature and usage of the options to commit.

[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>