transplant: fix crash if filter script munges log file
This fixes an UnboundLocalError crash if the filter script removes the
'User' or 'Date' lines from the log file.
$ mkdir t
$ cd t
$ hg init
$ echo a > a
$ hg add a
$ hg commit -m test
$ rm .hg/requires
$ hg tip
abort: index 00changelog.i unknown format 2!
[255]
$ echo indoor-pool > .hg/requires
$ hg tip
abort: requirement 'indoor-pool' not supported!
[255]