tests: scrub environment of GIT_* variables

Variables from the inherited environment that are meaningful to git
can break tests in undesirable ways.  For example,

	GIT_PAGER=more sh t5400-send-pack.sh -v -i

hangs.  So unset all environment variables in the GIT_ namespace in
test-lib, with a few exceptions:

- GIT_TRACE* are useful for tracking down bugs exhibited by a failing
  test;

- GIT_DEBUG* are GIT_TRACE variables by another name, practically
  speaking.  They should probably be tweaked to follow the
  GIT_TRACE_foo scheme and use trace_printf machinery some time.

- GIT_USE_LOOKUP from v1.5.6-rc0~134^2~1 (sha1-lookup: more memory
  efficient search in sorted list of SHA-1, 2007-12-29) is about
  trying an alternate implementation strategy rather than changing
  semantics and it can be useful to compare performance with and
  without it set.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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