get_sha1() shorthands for blob/tree objects

This is a fairly straightforward patch to allow "get_sha1()" to also have
shorthands for tree and blob objects.

The syntax is very simple and intuitive: you can specify a tree or a blob
by simply specifying <revision>:<path>, and get_sha1() will do the SHA1
lookup from the tree for you.

You can currently do it with "git ls-tree <rev> <path>" and parsing the
output, but that's actually pretty awkward.

With this, you can do something like

	git cat-file blob v1.2.4:Makefile

to get the contents of "Makefile" at revision v1.2.4.

Now, this isn't necessarily something you really need all that often, but
the concept itself is actually pretty powerful. We could, for example,
allow things like

	git diff v0.99.6:git-commit-script..v1.3.0:git-commit.sh

to see the difference between two arbitrary files in two arbitrary
revisions. To do that, the only thing we'd have to do is to make
git-diff-tree accept two blobs to diff, in addition to the two trees it
now expects.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

sha1_name.c

@@ -3,6 +3,7 @@
 #include "commit.h"
 #include "tree.h"
 #include "blob.h"
+#include "diff.h"
 
 static int find_short_object_filename(int len, const char *name, unsigned char *sha1)
 {
@@ -449,12 +450,76 @@ static int get_sha1_1(const char *name, int len, unsigned char *sha1)
 	return get_short_sha1(name, len, sha1, 0);
 }
 
+static int get_tree_entry(const unsigned char *, const char *, unsigned char *);
+
+static int find_tree_entry(struct tree_desc *t, const char *name, unsigned char *result)
+{
+	int namelen = strlen(name);
+	while (t->size) {
+		const char *entry;
+		const unsigned char *sha1;
+		int entrylen, cmp;
+		unsigned mode;
+
+		sha1 = tree_entry_extract(t, &entry, &mode);
+		update_tree_entry(t);
+		entrylen = strlen(entry);
+		if (entrylen > namelen)
+			continue;
+		cmp = memcmp(name, entry, entrylen);
+		if (cmp > 0)
+			continue;
+		if (cmp < 0)
+			break;
+		if (entrylen == namelen) {
+			memcpy(result, sha1, 20);
+			return 0;
+		}
+		if (name[entrylen] != '/')
+			continue;
+		if (!S_ISDIR(mode))
+			break;
+		if (++entrylen == namelen) {
+			memcpy(result, sha1, 20);
+			return 0;
+		}
+		return get_tree_entry(sha1, name + entrylen, result);
+	}
+	return -1;
+}
+
+static int get_tree_entry(const unsigned char *tree_sha1, const char *name, unsigned char *sha1)
+{
+	int retval;
+	void *tree;
+	struct tree_desc t;
+
+	tree = read_object_with_reference(tree_sha1, tree_type, &t.size, NULL);
+	if (!tree)
+		return -1;
+	t.buf = tree;
+	retval = find_tree_entry(&t, name, sha1);
+	free(tree);
+	return retval;
+}
+
 /*
  * This is like "get_sha1_basic()", except it allows "sha1 expressions",
  * notably "xyz^" for "parent of xyz"
  */
 int get_sha1(const char *name, unsigned char *sha1)
 {
+	int ret;
+
 	prepare_alt_odb();
-	return get_sha1_1(name, strlen(name), sha1);
+	ret = get_sha1_1(name, strlen(name), sha1);
+	if (ret < 0) {
+		const char *cp = strchr(name, ':');
+		if (cp) {
+			unsigned char tree_sha1[20];
+			if (!get_sha1_1(name, cp-name, tree_sha1))
+				return get_tree_entry(tree_sha1, cp+1, sha1);
+		}
+	}
+	return ret;
 }