Dgit.pm: reflog_cache_lookup: Replace `confess' with `die' after exec

Dgit.pm: reflog_cache_lookup: Replace `confess' with `die' after exec

With `confess' after `exec', perl complains

  Statement unlikely to be reached at /usr/share/perl5/Debian/Dgit.pm line 770.
	  (Maybe you meant system() when you said exec()?)

The error message's suggestion is not right here - what is wrong is
that confess is not really appropriate in the child in this way.

Replace it with die, using a better error string.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>