editor: move editor-related functions and declarations into common file

cache.h and strbuf.[ch] had editor-related functions.  Move these into
editor.[ch].

Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pager.c

@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "config.h"
+#include "editor.h"
 #include "run-command.h"
 #include "sigchain.h"
 #include "alias.h"