D5301: store: append to fncache if there are only new files to write

[Yuya Nishihara]

> @@ -479,32 +481,45 @@
>                  fp.write(encodedir('\n'.join(self.entries) + '\n'))
>              fp.close()
>              self._dirty = False
> +        if self.addls:
> +            # if we have just new entries, let's append them to the fncache
> +            tr.addbackup('fncache')
> +            fp = self.vfs('fncache', mode='ab', atomictemp=True)
> +            if self.addls:
> +                fp.write(encodedir('\n'.join(self.addls) + '\n'))
> +            fp.close()
> +            self.entries = None
> +            self.addls = set()

It's probably better to write `entries | addls` at once if there are both
adds and removes. Appending to an `atomictemp` file means the entire file
is copied first.