[RFC] get_sha1(): :path and :[0-3]:path to extract from index.

[Junio C Hamano <junkio@cox.net>]

[ NOTE! The reason I put "RFC" in the subject rather than "PATCH" is that 
  I'm not 100% sure this isn't just a "shiny object" of mine rather than a 
  really useful thing to do. What do people think? Have you ever wanted to 
  access individual files in some random revision? Do you think this is 
  useful? I think it's cool and _may_ be useful, but I'm not going to 
  really push this patch. Consider it a throw-away patch unless somebody 
  else finds it intriguing enough.. ]

This is a fairly straightforward patch to allow "get_sha1()" to
also have shorthands for blob objects in the current index.

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

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

With this, you can do something like

	git cat-file blob :3:Makefile

to get the contents of "Makefile" from their version during a
conflicted merge.

This does not do trees like <ent>:<path> version does.  We could
do that by writing out a temporary tree object if we wanted to,
but that would smudge the object database, so I refrained from
doing that as part of this patch.  If we did that,

	git diff-tree :2: :3:

would be an inefficient equivalent to "git diff-stages 2 3".

---
 sha1_name.c |   54 ++++++++++++++++++++++++++++++++++++++++++++++--------
 1 files changed, 46 insertions(+), 8 deletions(-)

diff --git a/sha1_name.c b/sha1_name.c
index 345935b..ec5cd2c 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -458,17 +458,55 @@ int get_sha1(const char *name, unsigned 
 {
 	int ret;
 	unsigned unused;
+	int namelen = strlen(name);
+	const char *cp;
 
 	prepare_alt_odb();
-	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,
-						      &unused);
+	ret = get_sha1_1(name, namelen, sha1);
+	if (!ret)
+		return ret;
+	/* sha1:path --> object name of path in ent sha1
+	 * :path -> object name of path in index
+	 * :[0-3]:path -> object name of path in index at stage
+	 */
+	if (name[0] == ':') {
+		int stage = 0;
+		struct cache_entry *ce;
+		int pos;
+		if (namelen < 3 ||
+		    name[2] != ':' ||
+		    name[1] < '0' || '3' < name[1])
+			cp = name + 1;
+		else {
+			stage = name[1] - '0';
+			cp = name + 3;
 		}
+		namelen = namelen - (cp - name);
+		if (!active_cache)
+			read_cache();
+		if (active_nr < 0)
+			return -1;
+		pos = cache_name_pos(cp, namelen);
+		if (pos < 0)
+			pos = -pos - 1;
+		while (pos < active_nr) {
+			ce = active_cache[pos];
+			if (ce_namelen(ce) != namelen ||
+			    memcmp(ce->name, cp, namelen))
+				break;
+			if (ce_stage(ce) == stage) {
+				memcpy(sha1, ce->sha1, 20);
+				return 0;
+			}
+		}
+		return -1;
+	}
+	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,
+					      &unused);
 	}
 	return ret;
 }
-- 
1.3.0.g6082