t1500: avoid setting environment variables outside of testses/t1500-modernize

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>

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