cache.h: remove this no-longer-used header

Since this header showed up in some places besides just #include
statements, update/clean-up/remove those other places as well.

Note that compat/fsmonitor/fsm-path-utils-darwin.c previously got
away with violating the rule that all files must start with an include
of git-compat-util.h (or a short-list of alternate headers that happen
to include it first).  This change exposed the violation and caused it
to stop building correctly; fix it by having it include
git-compat-util.h first, as per policy.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/merge.c b/merge.c
index fade6b2..83bcba6 100644
--- a/merge.c
+++ b/merge.c
@@ -1,4 +1,4 @@
-#include "cache.h"
+#include "git-compat-util.h"
 #include "diff.h"
 #include "diffcore.h"
 #include "gettext.h"