convert: make apply_filter() adhere to standard Git error handling

apply_filter() returns a boolean that tells the caller if it
"did convert or did not convert". The variable `ret` was used throughout
the function to track errors whereas `1` denoted success and `0`
failure. This is unusual for the Git source where `0` denotes success.

Rename the variable and flip its value to make the function easier
readable for Git developers.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: larsxschneider

convert.c

@@ -451,7 +451,7 @@ static int apply_filter(const char *path, const char *src, size_t len, int fd,
 	 *
 	 * (child --> cmd) --> us
 	 */
-	int ret = 1;
+	int err = 0;
 	struct strbuf nbuf = STRBUF_INIT;
 	struct async async;
 	struct filter_params params;
@@ -477,23 +477,20 @@ static int apply_filter(const char *path, const char *src, size_t len, int fd,
 		return 0;	/* error was already reported */
 
 	if (strbuf_read(&nbuf, async.out, len) < 0) {
-		error("read from external filter '%s' failed", cmd);
-		ret = 0;
+		err = error("read from external filter '%s' failed", cmd);
 	}
 	if (close(async.out)) {
-		error("read from external filter '%s' failed", cmd);
-		ret = 0;
+		err = error("read from external filter '%s' failed", cmd);
 	}
 	if (finish_async(&async)) {
-		error("external filter '%s' failed", cmd);
-		ret = 0;
+		err = error("external filter '%s' failed", cmd);
 	}
 
-	if (ret) {
+	if (!err) {
 		strbuf_swap(dst, &nbuf);
 	}
 	strbuf_release(&nbuf);
-	return ret;
+	return !err;
 }
 
 static struct convert_driver {