[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: NJunio C Hamano <junkio@cox.net>
Signed-off-by: NLinus 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

t/t1000-read-tree-m-3way.sh

@@ -464,6 +464,15 @@ test_expect_success \
      git-read-tree -m $tree_O $tree_A $tree_B &&
      check_result"
 
+test_expect_success \
+    '14 - may match B in O && A && B && O==A && O!=B case' \
+    "rm -f .git/index NM &&
+     cp .orig-B/NM NM &&
+     git-update-cache --add NM &&
+     echo extra >>NM &&
+     git-read-tree -m $tree_O $tree_A $tree_B &&
+     check_result"
+
 test_expect_failure \
     '14 (fail) - must match and be up-to-date in O && A && B && O==A && O!=B case' \
     "rm -f .git/index NM &&