alloc.h: move ALLOC_GROW() functions from cache.h

This allows us to replace includes of cache.h with includes of the much
smaller alloc.h in many places.  It does mean that we also need to add
includes of alloc.h in a number of C files.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.c

@@ -1,4 +1,5 @@
-#include "cache.h"
+#include "git-compat-util.h"
+#include "alloc.h"
 #include "lockfile.h"
 #include "tree.h"
 #include "tree-walk.h"