Add 'git-rev-parse' helper script

It's an incredibly cheesy helper that changes human-readable revision
arguments into the git-rev-list argument format.

You can use it to do something like this:

	git-rev-list --pretty $(git-rev-parse --default HEAD "$@")

which is what git-log-script will become. Here git-rev-parse will
then allow you to use arguments like "v2.6.12-rc5.." or similar
human-readable ranges.

It's really quite stupid: "a..b" will be converted into "a" and "^b" if
"a" and "b" are valid object pointers.  And the "--default" case will be
used if nothing but flags have been seen, so that you can default to a
certain argument if there are no other ranges.

rev-parse.c

@@ -0,0 +1,70 @@
+/*
+ * rev-parse.c
+ *
+ * Copyright (C) Linus Torvalds, 2005
+ */
+#include "cache.h"
+
+int main(int argc, char **argv)
+{
+	int i, as_is = 0;
+	char *def = NULL;
+	unsigned char sha1[20];
+
+	for (i = 1; i < argc; i++) {
+		char *arg = argv[i];
+		char *dotdot;
+	
+		if (as_is) {
+			printf("%s\n", arg);
+			continue;
+		}
+		if (*arg == '-') {
+			if (!strcmp(arg, "--")) {
+				if (def) {
+					printf("%s\n", def);
+					def = NULL;
+				}
+				as_is = 1;
+			}
+			if (!strcmp(arg, "--default")) {
+				if (def)
+					printf("%s\n", def);
+				def = argv[i+1];
+				i++;
+				continue;
+			}
+			printf("%s\n", arg);
+			continue;
+		}
+		def = NULL;
+		if (!get_sha1(arg, sha1)) {
+			printf("%s\n", sha1_to_hex(sha1));
+			continue;
+		}
+		if (*arg == '^' && !get_sha1(arg+1, sha1)) {
+			printf("^%s\n", sha1_to_hex(sha1));
+			continue;
+		}
+		dotdot = strstr(arg, "..");
+		if (dotdot) {
+			unsigned char end[20];
+			char *n = dotdot+2;
+			*dotdot = 0;
+			if (!get_sha1(arg, sha1)) {
+				if (!*n)
+					n = "HEAD";
+				if (!get_sha1(n, end)) {
+					printf("%s\n", sha1_to_hex(end));
+					printf("^%s\n", sha1_to_hex(sha1));
+					continue;
+				}
+			}
+			*dotdot = '.';
+		}
+		printf("%s\n", arg);
+	}
+	if (def)
+		printf("%s\n", def);
+	return 0;
+}