Use rpmfiConfigConflictIndex() for modified config testing on erase too

- Besides eliminating another copy-paste variant of the config file
  digest calculation and comparison, this fixes the case modified
  %config symlink gets silently erased. Unlike a regular config file,
  a symlink does not contain a whole lot of carefully tuned data, but
  if it's marked %config I think we should honor the %config file rules.
- Adjust test-suite expectations to match the new behavior.

lib/transaction.c

@@ -544,21 +544,9 @@ assert(otherFi != NULL);
 		break;
 		
 	    /* Pre-existing modified config files need to be saved. */
-	    if (S_ISREG(FMode) && (FFlags & RPMFILE_CONFIG)) {
-	    	int algo = 0;
-		size_t diglen = 0;
-		const unsigned char *digest;
-		if ((digest = rpmfiFDigestIndex(fi, i, &algo, &diglen))) {
-		    unsigned char fdigest[diglen];
-		    char *fn = rpmfiFNIndex(fi, i);
-		    int modified = (!rpmDoDigest(algo, fn, 0, fdigest, NULL) &&
-				    memcmp(digest, fdigest, diglen));
-		    free(fn);
-		    if (modified) {
-			rpmfsSetAction(fs, i, FA_BACKUP);
-			break;
-		    }
-		}
+	    if (rpmfiConfigConflictIndex(fi, i)) {
+		rpmfsSetAction(fs, i, FA_BACKUP);
+		break;
 	    }
 	
 	    /* Otherwise, we can just erase. */

tests/rpmconfig2.at

@@ -92,10 +92,8 @@ test ! -L "${cf}"
 AT_CLEANUP
 
 # Erase modified config link
-# Broken, no backup taken
 AT_SETUP([erase changed config link])
 AT_KEYWORDS([install])
-AT_XFAIL_IF([test $RPM_XFAIL -gt 0])
 AT_CHECK([
 RPMDB_CLEAR
 RPMDB_INIT