add is_dot_or_dotdot inline function

A new inline function is_dot_or_dotdot is used to check if the
directory name is either "." or "..". It returns a non-zero value if
the given string is "." or "..". It's applicable to a lot of Git
source code.

Signed-off-by: Alexander Potashev <aspotashev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/dir.c b/dir.c
index 0131983d..3347f46 100644
--- a/dir.c
+++ b/dir.c
@@ -585,10 +585,8 @@
 			int len, dtype;
 			int exclude;
 
-			if ((de->d_name[0] == '.') &&
-			    (de->d_name[1] == 0 ||
-			     !strcmp(de->d_name + 1, ".") ||
-			     !strcmp(de->d_name + 1, "git")))
+			if (is_dot_or_dotdot(de->d_name) ||
+			     !strcmp(de->d_name, ".git"))
 				continue;
 			len = strlen(de->d_name);
 			/* Ignore overly long pathnames! */
@@ -793,10 +791,8 @@
 	len = path->len;
 	while ((e = readdir(dir)) != NULL) {
 		struct stat st;
-		if ((e->d_name[0] == '.') &&
-		    ((e->d_name[1] == 0) ||
-		     ((e->d_name[1] == '.') && e->d_name[2] == 0)))
-			continue; /* "." and ".." */
+		if (is_dot_or_dotdot(e->d_name))
+			continue;
 
 		strbuf_setlen(path, len);
 		strbuf_addstr(path, e->d_name);