git-clean: honor core.excludesfile

git-clean did not honor core.excludesfile configuration
variable, although some other commands such as git-add and
git-status did.  Fix this inconsistency.

Original report and patch from Shun'ichi Fuji.  Rewritten by me
and bugs and tests are mine.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t7300-clean.sh b/t/t7300-clean.sh
index eb0847a..0ed4ae2 100755
--- a/t/t7300-clean.sh
+++ b/t/t7300-clean.sh
@@ -177,4 +177,15 @@
 
 '
 
+test_expect_success 'core.excludesfile' '
+
+	echo excludes >excludes &&
+	echo included >included &&
+	git config core.excludesfile excludes &&
+	output=$(git clean -n excludes included 2>&1) &&
+	expr "$output" : ".*included" >/dev/null &&
+	! expr "$output" : ".*excludes" >/dev/null
+
+'
+
 test_done