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>

builtin/commit.c

@@ -12,6 +12,7 @@
 #include "cache-tree.h"
 #include "color.h"
 #include "dir.h"
+#include "environment.h"
 #include "builtin.h"
 #include "diff.h"
 #include "diffcore.h"