tests: introduce test_unset_prereq, for debugging

While working on the --convert-graft-file test, I missed that I was
relying on the GPG prereq, by using output of test cases that were only
run under that prereq.

For debugging, it was really convenient to force that prereq to be
unmet, but there was no easy way to do that. So I came up with a way,
and this patch reflects the cleaned-up version of that way.

For convenience, the following two methods are now supported ways to
pretend that a prereq is not met:

	test_set_prereq !GPG

and

	test_unset_prereq GPG

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -278,8 +278,20 @@ write_script () {
 # The single parameter is the prerequisite tag (a simple word, in all
 # capital letters by convention).
 
+test_unset_prereq () {
+	! test_have_prereq "$1" ||
+	satisfied_prereq="${satisfied_prereq% $1 *} ${satisfied_prereq#* $1 }"
+}
+
 test_set_prereq () {
-	satisfied_prereq="$satisfied_prereq$1 "
+	case "$1" in
+	!*)
+		test_unset_prereq "${1#!}"
+		;;
+	*)
+		satisfied_prereq="$satisfied_prereq$1 "
+		;;
+	esac
 }
 satisfied_prereq=" "
 lazily_testable_prereq= lazily_tested_prereq=