cleanup: fix possible overflow errors in binary search

A common mistake when writing binary search is to allow possible
integer overflow by using the simple average:

	mid = (min + max) / 2;

Instead, use the overflow-safe version:

	mid = min + (max - min) / 2;

This translation is safe since the operation occurs inside a loop
conditioned on "min < max". The included changes were found using
the following git grep:

	git grep '/ *2;' '*.c'

Making this cleanup will prevent future review friction when a new
binary search is contructed based on existing code.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.c

@@ -49,7 +49,7 @@ static int subtree_pos(struct cache_tree *it, const char *path, int pathlen)
 	lo = 0;
 	hi = it->subtree_nr;
 	while (lo < hi) {
-		int mi = (lo + hi) / 2;
+		int mi = lo + (hi - lo) / 2;
 		struct cache_tree_sub *mdl = down[mi];
 		int cmp = subtree_name_cmp(path, pathlen,
 					   mdl->name, mdl->namelen);