t7003: Use test_commit instead of custom function

t7003: Use test_commit instead of custom function

t7003-filter-branch.sh had a make_commit() function that was identical
to test_commit() in test-lib.sh except that it used tr to create a
lowercase file name from the uppercase branch name instead of
appending ".t".

Not only is this unneeded code duplication, it also was something
simply waiting to fail on case-insensitive file systems.  So replace
all uses of make_commit with test_commit.

While we're editing the setup, chain it together with && so that
failures early in the sequence don't get lost and add a commit graph.

Signed-off-by: Brian Gernhardt <brian@gernhardtsoftware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>