rerere: do not leak mmfile[] for a path with multiple stage #1 An error occurred entries

A conflicted index can have multiple stage #1
  
An error occurred
 
 entries when dealing
with a criss-cross merge and using the "resolve" merge strategy.

Plug the leak by reading only the first one of the same stage
entries.

Strictly speaking, this fix does change the semantics, in that we
used to use the last stage #1
  
An error occurred
 
 entry as the common ancestor when
doing the plain-vanilla three-way merge, but with the leak fix, we
will use the first stage #1
  
An error occurred
 
 entry.  But it is not a grave backward
compatibility breakage.  Either way, we are arbitrarily picking one
of multiple stage #1
  
An error occurred
 
 entries and using it, ignoring others, and
there is no meaning in the ordering of these stage #1
  
An error occurred
 
 entries.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

rerere.c

@@ -337,8 +337,10 @@ static int handle_cache(const char *path, unsigned char *sha1, const char *outpu
 		if (ce_namelen(ce) != len || memcmp(ce->name, path, len))
 			break;
 		i = ce_stage(ce) - 1;
-		mmfile[i].ptr = read_sha1_file(ce->sha1, &type, &size);
-		mmfile[i].size = size;
+		if (!mmfile[i].ptr) {
+			mmfile[i].ptr = read_sha1_file(ce->sha1, &type, &size);
+			mmfile[i].size = size;
+		}
 	}
 	for (i = 0; i < 3; i++)
 		if (!mmfile[i].ptr && !mmfile[i].size)