test-lib-functions: mark 'test_commit' variables as 'local'

test-lib-functions: mark 'test_commit' variables as 'local'

Some variables in 'test_commit' have names that are common enough that
it is very likely that test authors might use them in a test. If they do
so and use 'test_commit' between setting such a variable and using it,
the variable value from 'test_commit' will leak back into the test and
most likely break it.

Prevent that by marking all variables in 'test_commit' as 'local'. This
allow a subsequent commit to use a 'tag' variable.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>