t1020: use --git-dir instead of subshell for bare repo

Replace an unnecessarily complex subshell pattern with a much simpler
`--git-dir`-based one. The latter is not only simpler, it also no
longer relies on implicit bare repo discovery, which would fail with
`safe.bareRepository=explicit`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

t/t1020-subdirectory.sh

@@ -177,10 +177,7 @@ test_expect_success 'no file/rev ambiguity check inside a bare repo (explicit GI
 test_expect_success 'no file/rev ambiguity check inside a bare repo' '
 	test_when_finished "rm -fr foo.git" &&
 	git clone -s --bare .git foo.git &&
-	(
-		cd foo.git &&
-		git show -s HEAD
-	)
+	git --git-dir=foo.git show -s HEAD
 '
 
 test_expect_success SYMLINKS 'detection should not be fooled by a symlink' '