Allow users to un-configure rename detection

I told people on the kernel mailing list to please use "-M" when sending
me rename patches, so that I can see what they do while reading email
rather than having to apply the patch and then look at the end result.

I also told them that if they want to make it the default, they can just
add

	[diff]
		renames

to their ~/.gitconfig file. And while I was thinking about that, I wanted
to also check whether you can then mark individual projects to _not_ have
that default in the per-repository .git/config file.

And you can't. Currently you cannot have a global "enable renames by
default" and then a local ".. but not for _this_ project". Why? Because if
somebody writes

	[diff]
		renames = no

we simply ignore it, rather than resetting "diff_detect_rename_default"
back to zero.

Fixed thusly.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -62,6 +62,15 @@ static int parse_diff_color_slot(const char *var, int ofs)
 	die("bad config variable '%s'", var);
 }
 
+static int git_config_rename(const char *var, const char *value)
+{
+	if (!value)
+		return DIFF_DETECT_RENAME;
+	if (!strcasecmp(value, "copies") || !strcasecmp(value, "copy"))
+		return  DIFF_DETECT_COPY;
+	return git_config_bool(var,value) ? DIFF_DETECT_RENAME : 0;
+}
+
 /*
  * These are to give UI layer defaults.
  * The core-level commands such as git-diff-files should
@@ -75,13 +84,7 @@ int git_diff_ui_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 	if (!strcmp(var, "diff.renames")) {
-		if (!value)
-			diff_detect_rename_default = DIFF_DETECT_RENAME;
-		else if (!strcasecmp(value, "copies") ||
-			 !strcasecmp(value, "copy"))
-			diff_detect_rename_default = DIFF_DETECT_COPY;
-		else if (git_config_bool(var,value))
-			diff_detect_rename_default = DIFF_DETECT_RENAME;
+		diff_detect_rename_default = git_config_rename(var, value);
 		return 0;
 	}
 	if (!strcmp(var, "diff.autorefreshindex")) {