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>

commit.c

@@ -2,6 +2,7 @@
 #include "tag.h"
 #include "commit.h"
 #include "commit-graph.h"
+#include "environment.h"
 #include "gettext.h"
 #include "hex.h"
 #include "repository.h"