[PATCH] read-tree: loosen too strict index requirements

This patch teaches read-tree 3-way merge that, when only "the
other tree" changed a path, and if the index file already has
the same change, we are not in a situation that would clobber
the index and the work tree, and lets the merge succeed; this is
case #14ALT in t1000 test.  It does not change the result of the
merge, but prevents it from failing when it does not have to.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

read-tree.c

@@ -280,6 +280,12 @@ static int threeway_merge(struct cache_entry *stages[4],
 		}
 		/* otherwise we will apply the original rule */
 	}
+	/* #14ALT */
+	if (a && b && c && same(a, b) && !same(a, c)) {
+		if (old && same(old, c))
+			return merged_entry_allow_dirty(c, old, dst);
+		/* otherwise the regular rule applies */
+	}
 	/*
 	 * If we have an entry in the index cache ("old"), then we want
 	 * to make sure that it matches any entries in stage 2 ("first