git: treat "git -C '<path>'" as a no-op when <path> is empty

'git -C ""' unhelpfully dies with error "Cannot change to ''",
whereas the shell treats `cd ""' as a no-op.  Taking the shell's
behavior as a precedent, teach git to treat `-C ""' as a no-op, as
well.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -204,10 +204,12 @@ static int handle_options(const char ***argv, int *argc, int *envchanged)
 				fprintf(stderr, "No directory given for -C.\n" );
 				usage(git_usage_string);
 			}
-			if (chdir((*argv)[1]))
-				die_errno("Cannot change to '%s'", (*argv)[1]);
-			if (envchanged)
-				*envchanged = 1;
+			if ((*argv)[1][0]) {
+				if (chdir((*argv)[1]))
+					die_errno("Cannot change to '%s'", (*argv)[1]);
+				if (envchanged)
+					*envchanged = 1;
+			}
 			(*argv)++;
 			(*argc)--;
 		} else {

t/t0056-git-C.sh

@@ -14,6 +14,16 @@ test_expect_success '"git -C <path>" runs git from the directory <path>' '
 	test_cmp expected actual
 '
 
+test_expect_success '"git -C <path>" with an empty <path> is a no-op' '
+	(
+		mkdir -p dir1/subdir &&
+		cd dir1/subdir &&
+		git -C "" rev-parse --show-prefix >actual &&
+		echo subdir/ >expect &&
+		test_cmp expect actual
+	)
+'
+
 test_expect_success 'Multiple -C options: "-C dir1 -C dir2" is equivalent to "-C dir1/dir2"' '
 	test_create_repo dir1/dir2 &&
 	echo 1 >dir1/dir2/b.txt &&