init: handle empty "template" parameter

If a user passes "--template=", then our template parameter
is blank. Unfortunately, copy_templates() assumes it has at
least one character, and does all sorts of bad things like
reading from template[-1] and then proceeding to link all of
'/' into the .git directory.

This patch just checks for that condition in copy_templates
and aborts. As a side effect, this means that --template=
now has the meaning "don't copy any templates."

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-init-db.c

@@ -127,6 +127,8 @@ static void copy_templates(const char *template_dir)
 			template_dir = strbuf_detach(&d, NULL);
 		}
 	}
+	if (!template_dir[0])
+		return;
 	strcpy(template_path, template_dir);
 	template_len = strlen(template_path);
 	if (template_path[template_len-1] != '/') {

t/t0001-init.sh

@@ -141,4 +141,30 @@ test_expect_success 'reinit' '
 	test_cmp again/empty again/err2
 '
 
+test_expect_success 'init with --template' '
+	mkdir template-source &&
+	echo content >template-source/file &&
+	(
+		mkdir template-custom &&
+		cd template-custom &&
+		git init --template=../template-source
+	) &&
+	test_cmp template-source/file template-custom/.git/file
+'
+
+test_expect_success 'init with --template (blank)' '
+	(
+		mkdir template-plain &&
+		cd template-plain &&
+		git init
+	) &&
+	test -f template-plain/.git/info/exclude &&
+	(
+		mkdir template-blank &&
+		cd template-blank &&
+		git init --template=
+	) &&
+	! test -f template-blank/.git/info/exclude
+'
+
 test_done