Make "cache_name_pos()" available to others.

It finds the cache entry position for a given name, and is
generally useful. Sure, everybody can just scan the active
cache array, but since it's sorted, you actually want to
search it with a binary search, so let's not duplicate that
logic all over the place.
diff --git a/cache.h b/cache.h
index 900824a..a23ad51 100644
--- a/cache.h
+++ b/cache.h
@@ -73,6 +73,7 @@
 
 /* Initialize the cache information */
 extern int read_cache(void);
+extern int cache_name_pos(const char *name, int namelen);
 
 /* Return a statically allocated filename matching the sha1 signature */
 extern char *sha1_file_name(unsigned char *sha1);