tree: make path len uint16_t and avoid holes

This reduces the size of the struct from 32 to 26 bytes, and leaves a
single padding byte at the end of the struct (which comes from the
zero-length array).

src/tree.c

@@ -125,7 +125,7 @@ static git_tree_entry *alloc_entry(const char *filename)
 
 struct tree_key_search {
 	const char *filename;
-	size_t filename_len;
+	uint16_t filename_len;
 };
 
 static int homing_search_cmp(const void *key, const void *array_member)

src/tree.h

@@ -16,9 +16,9 @@
 
 struct git_tree_entry {
 	uint16_t attr;
+	uint16_t filename_len;
 	git_oid oid;
 	bool pooled;
-	size_t filename_len;
 	char filename[GIT_FLEX_ARRAY];
 };