patchchdir() into list_commands() dir instead of building paths for stat()

diff --git a/help.c b/help.c
index 950f62d..906f8f6 100644
--- a/help.c
+++ b/help.c
@@ -96,35 +96,23 @@ static void pretty_print_string_list(struct cmdname **cmdname, int longest)
 static void list_commands(const char *exec_path, const char *prefix)
 {
 	unsigned int longest = 0;
-	char path[PATH_MAX];
-	int dirlen;
 	int prefix_len = strlen(prefix);
 	DIR *dir = opendir(exec_path);
 	struct dirent *de;
 
-	if (!dir) {
+	if (!dir || chdir(exec_path)) {
 		fprintf(stderr, "git: '%s': %s\n", exec_path, strerror(errno));
 		exit(1);
 	}
 
-	dirlen = strlen(exec_path);
-	if (PATH_MAX - 20 < dirlen) {
-		fprintf(stderr, "git: insanely long exec-path '%s'\n",
-			exec_path);
-		exit(1);
-	}
-
-	memcpy(path, exec_path, dirlen);
-	path[dirlen++] = '/';
-
 	while ((de = readdir(dir)) != NULL) {
 		struct stat st;
 		int entlen;
 
 		if (prefixcmp(de->d_name, prefix))
 			continue;
-		strcpy(path+dirlen, de->d_name);
-		if (stat(path, &st) || /* stat, not lstat */
+
+		if (stat(de->d_name, &st) || /* stat, not lstat */
 		    !S_ISREG(st.st_mode) ||
 		    !(st.st_mode & S_IXUSR))
 			continue;