t1500: avoid setting environment variables outside of tests

t1500: avoid setting environment variables outside of tests

Ideally, each test should be responsible for setting up state it needs
rather than relying upon transient global state. Toward this end, teach
test_rev_parse() to accept a "-g <dir>" option to allow callers to
specify the value of the GIT_DIR environment variable explicitly. Take
advantage of this new option to avoid polluting the global scope with
GIT_DIR assignments.

Implementation note: Typically, tests avoid polluting the global state
by wrapping transient environment variable assignments within a
subshell, however, this technique doesn't work here since test_config()
and test_unconfig() need to know GIT_DIR, as well, but neither function
can be used within a subshell. Consequently, GIT_DIR is instead cleared
manually via test_when_finished().

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>