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

t/test-lib.sh

@@ -43,29 +43,16 @@ TERM=dumb
 export LANG LC_ALL PAGER TERM TZ
 EDITOR=:
 unset VISUAL
-unset GIT_EDITOR
 unset EMAIL
-unset GIT_ALTERNATE_OBJECT_DIRECTORIES
-unset GIT_AUTHOR_DATE
+unset $(perl -e '
+	my @env = keys %ENV;
+	my @vars = grep(/^GIT_/ && !/^GIT_(TRACE|DEBUG|USE_LOOKUP)/, @env);
+	print join("\n", @vars);
+')
 GIT_AUTHOR_EMAIL=author@example.com
 GIT_AUTHOR_NAME='A U Thor'
-unset GIT_COMMITTER_DATE
 GIT_COMMITTER_EMAIL=committer@example.com
 GIT_COMMITTER_NAME='C O Mitter'
-unset GIT_DIFF_OPTS
-unset GIT_DIR
-unset GIT_WORK_TREE
-unset GIT_EXTERNAL_DIFF
-unset GIT_INDEX_FILE
-unset GIT_OBJECT_DIRECTORY
-unset GIT_CEILING_DIRECTORIES
-unset GIT_NOTES_REF
-unset GIT_NOTES_DISPLAY_REF
-unset GIT_NOTES_REWRITE_REF
-unset GIT_NOTES_REWRITE_MODE
-unset GIT_REFLOG_ACTION
-unset GIT_CHERRY_PICK_HELP
-unset GIT_QUIET
 GIT_MERGE_VERBOSITY=5
 export GIT_MERGE_VERBOSITY
 export GIT_AUTHOR_EMAIL GIT_AUTHOR_NAME