ls-files: pass prefix length explicitly to prune_cache()

The function prune_cache() relies on the fact that it is only called on
max_prefix and sneakily uses the matching global variable max_prefix_len
directly.  Tighten its interface by passing both the string and its
length as parameters.  While at it move the NULL check into the function
to collect all cache-pruning related logic in one place.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Reviewed-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/ls-files.c

@@ -369,11 +369,14 @@ static void show_files(struct dir_struct *dir)
 /*
  * Prune the index to only contain stuff starting with "prefix"
  */
-static void prune_cache(const char *prefix)
+static void prune_cache(const char *prefix, size_t prefixlen)
 {
-	int pos = cache_name_pos(prefix, max_prefix_len);
+	int pos;
 	unsigned int first, last;
 
+	if (!prefix)
+		return;
+	pos = cache_name_pos(prefix, prefixlen);
 	if (pos < 0)
 		pos = -pos-1;
 	memmove(active_cache, active_cache + pos,
@@ -384,7 +387,7 @@ static void prune_cache(const char *prefix)
 	while (last > first) {
 		int next = (last + first) >> 1;
 		const struct cache_entry *ce = active_cache[next];
-		if (!strncmp(ce->name, prefix, max_prefix_len)) {
+		if (!strncmp(ce->name, prefix, prefixlen)) {
 			first = next+1;
 			continue;
 		}
@@ -641,8 +644,7 @@ int cmd_ls_files(int argc, const char **argv, const char *cmd_prefix)
 	      show_killed || show_modified || show_resolve_undo))
 		show_cached = 1;
 
-	if (max_prefix)
-		prune_cache(max_prefix);
+	prune_cache(max_prefix, max_prefix_len);
 	if (with_tree) {
 		/*
 		 * Basic sanity check; show-stages and show-unmerged