environment: move comment_line_char from cache.h

This is one step towards making strbuf.c not depend upon cache.h.
Additional steps will follow in subsequent commits.

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

fmt-merge-msg.c

@@ -1,6 +1,7 @@
 #include "cache.h"
 #include "alloc.h"
 #include "config.h"
+#include "environment.h"
 #include "refs.h"
 #include "object-store.h"
 #include "diff.h"