color: use git_colorbool enum type to store colorbools

We traditionally used "int" to store and pass around the values defined
by "enum git_colorbool" (which were originally just #define macros).
Using an int doesn't produce incorrect results, but using the actual
enum makes the intent of the code more clear.

It would be nice if the compiler could catch cases where we used the
enum and an int interchangeably, since it's very easy to accidentally
check the boolean true/false of a colorbool like:

  if (branch_use_color)

This is wrong because GIT_COLOR_UNKNOWN and GIT_COLOR_AUTO evaluate to
true in C, even though we may ultimately decide not to use color. But C
is pretty happy to convert between ints and enums (even with various
-Wenum-* warnings). So this sadly doesn't protect us from such mistakes,
but it hopefully does make the code easier to read.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

log-tree.h

@@ -1,6 +1,8 @@
 #ifndef LOG_TREE_H
 #define LOG_TREE_H
 
+#include "color.h"
+
 struct rev_info;
 
 struct log_info {
@@ -26,7 +28,7 @@ int log_tree_diff_flush(struct rev_info *);
 int log_tree_commit(struct rev_info *, struct commit *);
 void show_log(struct rev_info *opt);
 void format_decorations(struct strbuf *sb, const struct commit *commit,
-			int use_color, const struct decoration_options *opts);
+			enum git_colorbool use_color, const struct decoration_options *opts);
 void show_decorations(struct rev_info *opt, struct commit *commit);
 void log_write_email_headers(struct rev_info *opt, struct commit *commit,
 			     char **extra_headers_p,