HPCC-9664 dfu copy of files generating odd names

Move StringBuffer for logical file name outside the local block to keep
its content.

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>

Added comment

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge branch 'HPCC-9664-fix' into candidate-4.0.0

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>