[PATCH] Don't update unchanged merge entries

In commit 34110cd4e394e3f92c01a4709689b384c34645d8 ("Make 'unpack_trees()' 
have a separate source and destination index") I introduced a really 
stupid bug in that it would always add merged entries with the CE_UPDATE 
flag set. That caused us to always re-write the file, even when it was 
already up-to-date in the source index.

Not only is that really stupid from a performance angle, but more 
importantly it's actively wrong: if we have dirty state in the tree when 
we merge, overwriting it with the result of the merge will incorrectly 
overwrite that dirty state.

This trivially fixes the problem - simply don't set the CE_UPDATE flag 
when the merge result matches the old state.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---

Ok, that was a really stupid one. 

On Sun, 16 Mar 2008, Linus Torvalds wrote:

Nope, I bisected it down to

	34110cd4e394e3f92c01a4709689b384c34645d8 is first bad commit

	Make 'unpack_trees()' have a separate source and destination index

and I'm trying to figure out what part of that triggered this bug.

 unpack-trees.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/unpack-trees.c b/unpack-trees.c
index 0cdf198..46d4f6c 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c

@@ -593,6 +593,8 @@ static int verify_absent(struct cache_entry *ce, const char *action,
 static int merged_entry(struct cache_entry *merge, struct cache_entry *old,
 		struct unpack_trees_options *o)
 {
+	int update = CE_UPDATE;
+
 	if (old) {
 		/*
 		 * See if we can re-use the old CE directly?

@@ -603,6 +605,7 @@ static int merged_entry(struct cache_entry *merge, struct cache_entry *old,
 		 */
 		if (same(old, merge)) {
 			copy_cache_entry(merge, old);
+			update = 0;
 		} else {
 			if (verify_uptodate(old, o))
 				return -1;

@@ -615,7 +618,7 @@ static int merged_entry(struct cache_entry *merge, struct cache_entry *old,
 		invalidate_ce_path(merge, o);
 	}
 
-	add_entry(o, merge, CE_UPDATE, CE_STAGEMASK);
+	add_entry(o, merge, update, CE_STAGEMASK);
 	return 1;
 }