Make rev-parse understand "extended sha1" syntax

You can say "HEAD.p" for the "parent of HEAD". It nests, so

	HEAD.p2.p

means parent of second parent of HEAD (which obviously depends
on HEAD being a merge).

rev-parse.c

@@ -4,6 +4,9 @@
  * Copyright (C) Linus Torvalds, 2005
  */
 #include "cache.h"
+#include "commit.h"
+
+static int get_extended_sha1(char *name, unsigned char *sha1);
 
 /*
  * Some arguments are relevant "revision" arguments,
@@ -32,6 +35,56 @@ static int is_rev_argument(const char *arg)
 	}
 }
 
+static int get_parent(char *name, unsigned char *result, int idx)
+{
+	unsigned char sha1[20];
+	int ret = get_extended_sha1(name, sha1);
+	struct commit *commit;
+	struct commit_list *p;
+
+	if (ret)
+		return ret;
+	commit = lookup_commit_reference(sha1);
+	if (!commit)
+		return -1;
+	if (parse_commit(commit))
+		return -1;
+	p = commit->parents;
+	while (p) {
+		if (!--idx) {
+			memcpy(result, p->item->object.sha1, 20);
+			return 0;
+		}
+		p = p->next;
+	}
+	return -1;
+}
+
+/*
+ * This is like "get_sha1()", except it allows "sha1 expressions",
+ * notably "xyz.p" for "parent of xyz"
+ */
+static int get_extended_sha1(char *name, unsigned char *sha1)
+{
+	int parent;
+	int len = strlen(name);
+
+	parent = 1;
+	if (len > 3 && name[len-1] >= '1' && name[len-1] <= '9') {
+		parent = name[len-1] - '0';
+		len--;
+	}
+	if (len > 2 && !memcmp(name + len - 2, ".p", 2)) {
+		int ret;
+		name[len-2] = 0;
+		ret = get_parent(name, sha1, parent);
+		name[len-2] = '.';
+		if (!ret)
+			return 0;
+	}
+	return get_sha1(name, sha1);
+}
+
 int main(int argc, char **argv)
 {
 	int i, as_is = 0, revs_only = 0, no_revs = 0;
@@ -83,10 +136,10 @@ int main(int argc, char **argv)
 			unsigned char end[20];
 			char *n = dotdot+2;
 			*dotdot = 0;
-			if (!get_sha1(arg, sha1)) {
+			if (!get_extended_sha1(arg, sha1)) {
 				if (!*n)
 					n = "HEAD";
-				if (!get_sha1(n, end)) {
+				if (!get_extended_sha1(n, end)) {
 					if (no_revs)
 						continue;
 					def = NULL;
@@ -97,14 +150,14 @@ int main(int argc, char **argv)
 			}
 			*dotdot = '.';
 		}
-		if (!get_sha1(arg, sha1)) {
+		if (!get_extended_sha1(arg, sha1)) {
 			if (no_revs)
 				continue;
 			def = NULL;
 			printf("%s\n", sha1_to_hex(sha1));
 			continue;
 		}
-		if (*arg == '^' && !get_sha1(arg+1, sha1)) {
+		if (*arg == '^' && !get_extended_sha1(arg+1, sha1)) {
 			if (no_revs)
 				continue;
 			def = NULL;