git-rev-parse: add "--not" flag to mark subsequent heads negative

If you have two lists of heads, and you want to see ones reachable from
list $a but not from list $b, just do

	git-rev-list $(git-rev-parse $a --not $b)

which is useful for both bisecting (where "b" would be the list of known
good revisions, and "a" would be the latest found bad head) and for just
seeing what the difference between two sets of heads are if you want to
generate a pack-file for the difference.

rev-parse.c

@@ -13,6 +13,10 @@ static int revs_only = 0;
 static int do_rev_argument = 1;
 static int output_revs = 0;
 
+#define NORMAL 0
+#define REVERSED 1
+static int show_type = NORMAL;
+
 static int get_extended_sha1(char *name, unsigned char *sha1);
 
 /*
@@ -42,20 +46,12 @@ static int is_rev_argument(const char *arg)
 	}
 }
 
-static void show_rev(unsigned char *sha1)
+static void show_rev(int type, unsigned char *sha1)
 {
 	if (no_revs)
 		return;
 	output_revs++;
-	puts(sha1_to_hex(sha1));
-}
-
-static void show_antirev(unsigned char *sha1)
-{
-	if (no_revs)
-		return;
-	output_revs++;
-	printf("^%s\n", sha1_to_hex(sha1));
+	printf("%s%s\n", type == show_type ? "" : "^", sha1_to_hex(sha1));
 }
 
 static void show_rev_arg(char *rev)
@@ -139,7 +135,7 @@ static void show_default(void)
 
 		def = NULL;
 		if (!get_extended_sha1(s, sha1)) {
-			show_rev(sha1);
+			show_rev(NORMAL, sha1);
 			return;
 		}
 		show_arg(s);
@@ -185,6 +181,10 @@ int main(int argc, char **argv)
 				single_rev = 1;
 				continue;
 			}
+			if (!strcmp(arg, "--not")) {
+				show_type ^= REVERSED;
+				continue;
+			}
 			show_arg(arg);
 			continue;
 		}
@@ -200,8 +200,8 @@ int main(int argc, char **argv)
 					if (no_revs)
 						continue;
 					def = NULL;
-					show_rev(end);
-					show_antirev(sha1);
+					show_rev(NORMAL, end);
+					show_rev(REVERSED, sha1);
 					continue;
 				}
 			}
@@ -211,14 +211,14 @@ int main(int argc, char **argv)
 			if (no_revs)
 				continue;
 			def = NULL;
-			show_rev(sha1);
+			show_rev(NORMAL, sha1);
 			continue;
 		}
 		if (*arg == '^' && !get_extended_sha1(arg+1, sha1)) {
 			if (no_revs)
 				continue;
 			def = NULL;
-			show_antirev(sha1);
+			show_rev(REVERSED, sha1);
 			continue;
 		}
 		show_default();