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>

builtin/commit.c

@@ -13,6 +13,7 @@
 #include "cache-tree.h"
 #include "color.h"
 #include "dir.h"
+#include "editor.h"
 #include "environment.h"
 #include "builtin.h"
 #include "diff.h"