t: refactor environment sanitization to not use Perl

Before executing tests we first sanitize the environment. Part of the
sanitization is to unset a couple of environment variables that we know
will change the behaviour of Git. This is done with a small Perl script,
which has the consequence that having a Perl interpreter available is a
strict requirement for running our unit tests.

The logic itself isn't particularly involved: we simply unset every
environment variable whose key starts with 'GIT_', but then explicitly
allow a subset of these.

Refactor the logic to instead use sed(1) so that it becomes possible to
execute our tests without Perl.

Based-on-patch-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -499,24 +499,20 @@ EDITOR=:
 # /usr/xpg4/bin/sh and /bin/ksh to bail out.  So keep the unsets
 # deriving from the command substitution clustered with the other
 # ones.
-unset VISUAL EMAIL LANGUAGE $("$PERL_PATH" -e '
+unset VISUAL EMAIL LANGUAGE $(env | sed -n \
-	my @env = keys %ENV;
+	-e '/^GIT_TRACE/d' \
-	my $ok = join("|", qw(
+	-e '/^GIT_DEBUG/d' \
-		TRACE
+	-e '/^GIT_TEST/d' \
-		DEBUG
+	-e '/^GIT_.*_TEST/d' \
-		TEST
+	-e '/^GIT_PROVE/d' \
-		.*_TEST
+	-e '/^GIT_VALGRIND/d' \
-		PROVE
+	-e '/^GIT_UNZIP/d' \
-		VALGRIND
+	-e '/^GIT_PERF_/d' \
-		UNZIP
+	-e '/^GIT_CURL_VERBOSE/d' \
-		PERF_
+	-e '/^GIT_TRACE_CURL/d' \
-		CURL_VERBOSE
+	-e '/^GIT_BUILD_DIR/d' \
-		TRACE_CURL
+	-e 's/^\(GIT_[^=]*\)=.*/\1/p'
-		BUILD_DIR
+)
-	));
-	my @vars = grep(/^GIT_/ && !/^GIT_($ok)/o, @env);
-	print join("\n", @vars);
-')
 unset XDG_CACHE_HOME
 unset XDG_CONFIG_HOME
 unset GITPERLLIB