treewide: remove unnecessary includes of cache.h

The last several commits were geared at replacing the include of cache.h
in strbuf.c with an include of git-compat-util.h.  Unfortunately, I had
to drop a patch moving some functions from cache.h to object-name.h, due
to excessive conflicts with other in-flight topics.

However, even without that patch, the series of patches so far allows us
to modify a number of C files to replace an include of cache.h with
git-compat-util.h.  Do that to reduce our dependencies.

(If we could have kept our object-name.h patch in this series, it would
have also let us reduce the includes in checkout.c and fmt-merge-msg.c
in addition to strbuf.c).

Just to ensure that nothing else was bringing in cache.h, all of the
affected files have been checked to ensure that
    gcc -E -I. $SOURCE_FILE | grep '"cache.h"'
found no hits and that
    make DEVELOPER=1 ${OBJECT_FILE_FOR_SOURCE_FILE}
successfully compiles without warnings.

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

gpg-interface.c

@@ -1,4 +1,4 @@
-#include "cache.h"
+#include "git-compat-util.h"
 #include "commit.h"
 #include "config.h"
 #include "gettext.h"