[PATCH] Add -u option to diff-cache to show UNCHANGED files

With -u diff-cache shows unchanged files, instead of files that changed. 
This is useful to implement a "git clean" command that throws away all 
checked out files that have not changed (yes, I really would like to 
have that). One could also do show-files and then substract the 
diff-cache output from it, but thats slow and clumsy. 

Signed-off-by: Andreas Gal <gal@uci.edu>

--- b407d753e520fa0b1523d770d98b3015af197275/diff-cache.c
+++ diff-cache.c
@@ -2,6 +2,7 @@
 
 static int cached_only = 0;
 static int line_termination = '\n';
+static int show_unchanged = 0;
 
 /* A file entry went away or appeared */
 static void show_file(const char *prefix, struct cache_entry *ce)
@@ -32,7 +33,7 @@
 	}
 
 	oldmode = ntohl(old->ce_mode);
-	if (mode == oldmode && !memcmp(sha1, old->sha1, 20))
+	if ((mode == oldmode && !memcmp(sha1, old->sha1, 20)) != show_unchanged)
 		return 0;
 
 	strcpy(old_sha1_hex, sha1_to_hex(old->sha1));
@@ -85,7 +86,7 @@
 	}
 }
 
-static char *diff_cache_usage = "diff-cache [-r] [-z] [--cached] <tree sha1>";
+static char *diff_cache_usage = "diff-cache [-r] [-u] [-z] [--cached] <tree sha1>";
 
 int main(int argc, char **argv)
 {
@@ -102,6 +103,10 @@
 			/* We accept the -r flag just to look like diff-tree */
 			continue;
 		}
+		if (!strcmp(arg, "-u")) {
+			show_unchanged = 1;
+			continue;
+		}
 		if (!strcmp(arg, "-z")) {
 			line_termination = '\0';
 			continue;
-
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