lstat_cache(): swap func(length, string) into func(string, length)

Swap function argument pair (length, string) into (string, length) to
conform with the commonly used order inside the GIT source code.

Also, add a note about this fact into the coding guidelines.

Signed-off-by: Kjetil Barvik <barvik@broadpark.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -129,3 +129,6 @@ For C programs:
    used in the git core command set (unless your command is clearly
    separate from it, such as an importer to convert random-scm-X
    repositories to git).
+
+ - When we pass <string, length> pair to functions, we should try to
+   pass them in that order.