Don't default index entries to "VALID"

The "VALID" flag in git refers to the "assume-unchanged" feature, such
that if, in a non-bare repository, an index entry is marked as "VALID",
the git CLI (e.g. `git status`) will assume that the file never changes,
and ignore any changes that do occur.

See #636 for the issue and
research leading to this change.

Author: jasonroelofs

ext/rugged/rugged_index.c

@@ -624,8 +624,6 @@ static void rb_git_indexentry_toC(git_index_entry *entry, VALUE rb_entry)
 		entry->flags &= ~GIT_IDXENTRY_VALID;
 		if (rugged_parse_bool(val))
 			entry->flags |= GIT_IDXENTRY_VALID;
-	} else {
-		entry->flags |= GIT_IDXENTRY_VALID;
 	}
 }