merge-recursive: fix remainder of was_dirty() to use original index

was_dirty() uses was_tracked(), which has been updated to use the original
index rather than the current one.  However, was_dirty() also had a
separate call to cache_file_exists(), causing it to still implicitly use
the current index.  Update that to instead use index_file_exists().

Also, was_dirty() had a hack where it would mark any file as non-dirty if
we simply didn't know its modification time.  This was due to using the
current index rather than the original index, because D/F conflicts and
such would cause unpack_trees() to not copy the modification times from
the original index to the current one.  Now that we are using the original
index, we can dispense with this hack.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -842,9 +842,9 @@ static int was_dirty(struct merge_options *o, const char *path)
 	if (o->call_depth || !was_tracked(o, path))
 		return !dirty;
 
-	ce = cache_file_exists(path, strlen(path), ignore_case);
-	dirty = (ce->ce_stat_data.sd_mtime.sec > 0 &&
-		 verify_uptodate(ce, &o->unpack_opts) != 0);
+	ce = index_file_exists(o->unpack_opts.src_index,
+			       path, strlen(path), ignore_case);
+	dirty = verify_uptodate(ce, &o->unpack_opts) != 0;
 	return dirty;
 }