checkout -f failed to check out a file if an existing directory interfered.

When path foo/bar existed in the working tree, checkout -f to switch to
a branch that has a file foo silently did a wrong thing.  It failed to
remove the directory foo, did not check out the file foo, and the worst
of all it did not report any errors.

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-read-tree.c

@@ -507,7 +507,7 @@ static int merged_entry(struct cache_entry *merge, struct cache_entry *old)
 	}
 
 	merge->ce_flags &= ~htons(CE_STAGEMASK);
-	add_cache_entry(merge, ADD_CACHE_OK_TO_ADD);
+	add_cache_entry(merge, ADD_CACHE_OK_TO_ADD|ADD_CACHE_OK_TO_REPLACE);
 	return 1;
 }
 
@@ -518,7 +518,7 @@ static int deleted_entry(struct cache_entry *ce, struct cache_entry *old)
 	else
 		verify_absent(ce->name, "removed");
 	ce->ce_mode = 0;
-	add_cache_entry(ce, ADD_CACHE_OK_TO_ADD);
+	add_cache_entry(ce, ADD_CACHE_OK_TO_ADD|ADD_CACHE_OK_TO_REPLACE);
 	invalidate_ce_path(ce);
 	return 1;
 }