Fix work-tree related breakages

In set_work_tree(), variable rel needs to be reinitialized to
NULL on every call (it should not be static).

Make sure the incoming dir variable is not too long before
copying to the temporary buffer, and make sure chdir to the
resulting directory succeeds.

This was spotted and fixed by Alex and Johannes in a handful
patch exchanges.  Here is the final version.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>

setup.c

@@ -201,26 +201,32 @@ int is_inside_work_tree(void)
  */
 const char *set_work_tree(const char *dir)
 {
-	char dir_buffer[PATH_MAX];
-	static char buffer[PATH_MAX + 1], *rel = NULL;
-	int len, postfix_len = strlen(DEFAULT_GIT_DIR_ENVIRONMENT) + 1;
+	char dir_buffer[PATH_MAX], *rel = NULL;
+	static char buffer[PATH_MAX + 1];
+	int len, suffix_len = strlen(DEFAULT_GIT_DIR_ENVIRONMENT) + 1;
 
 	/* strip the variable 'dir' of the postfix "/.git" if it has it */
 	len = strlen(dir);
-	if (len > postfix_len && !strcmp(dir + len - postfix_len,
-				"/" DEFAULT_GIT_DIR_ENVIRONMENT)) {
-			strncpy(dir_buffer, dir, len - postfix_len);
+	if (len > suffix_len &&
+	    !strcmp(dir + len - suffix_len, "/" DEFAULT_GIT_DIR_ENVIRONMENT)) {
+		if ((len - suffix_len) >= sizeof(dir_buffer))
+			die("directory name too long");
+		memcpy(dir_buffer, dir, len - suffix_len);
+		dir_buffer[len - suffix_len] = '\0';
 
 		/* are we inside the default work tree? */
 		rel = get_relative_cwd(buffer, sizeof(buffer), dir_buffer);
 	}
+
 	/* if rel is set, the cwd is _not_ the current working tree */
 	if (rel && *rel) {
 		if (!is_absolute_path(dir))
 			set_git_dir(make_absolute_path(dir));
 		dir = dir_buffer;
-		chdir(dir);
-		strcat(rel, "/");
+		if (chdir(dir))
+			die("cannot chdir to %s: %s", dir, strerror(errno));
+		else
+			strcat(rel, "/");
 		inside_git_dir = 0;
 	} else {
 		rel = NULL;