merge-file: correctly open files when in a subdir

run_setup_gently() is called before merge-file. This may result in changing
current working directory, which wasn't taken into account when opening a file
for writing.

Fix by prepending the passed prefix. Previous var is left so that error
messages keep referring to the file from the user's working directory
perspective.

Signed-off-by: Aleksander Boruch-Gruszecki <aleksander.boruchgruszecki@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge-file.c

@@ -90,7 +90,8 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 
 	if (ret >= 0) {
 		const char *filename = argv[0];
-		FILE *f = to_stdout ? stdout : fopen(filename, "wb");
+		const char *fpath = prefix_filename(prefix, prefixlen, argv[0]);
+		FILE *f = to_stdout ? stdout : fopen(fpath, "wb");
 
 		if (!f)
 			ret = error("Could not open %s for writing", filename);

t/t6023-merge-file.sh

@@ -69,7 +69,8 @@ test_expect_success 'works in subdirectory' '
 	cp new1.txt dir/a.txt &&
 	cp orig.txt dir/o.txt &&
 	cp new2.txt dir/b.txt &&
-	( cd dir && git merge-file a.txt o.txt b.txt )
+	( cd dir && git merge-file a.txt o.txt b.txt ) &&
+	test_path_is_missing a.txt
 '
 
 cp new1.txt test.txt