t7502: perform commits using alternate editor in a subshell

These tests call test_set_editor to set an alternate editor script, but
they appear to presume that the assignment is of a temporary nature and
will not have any effect outside of each individual test.  That is not
the case.  All of the test functions within a test script share a single
environment, so any variables modified in one, are visible in the ones
that follow.

So, let's protect the test functions that follow these, which set an
alternate editor, by performing the test_set_editor and 'git commit'
in a subshell.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>