Regression: paths to source files in putty + gcc logs are cut off

Paths to source files miss the first few characters.

A regression, current head 0.4.10-dev-357-g2158f31 Win x86 gcc is affected.

for example like in this log

the guilty rev is 0.4.10-dev-289-g79ad68ddc3a47f55e348af5b9d47242b5ca72877 by Timo Kreuzer

only Windows gcc builds are affected, but no Linux gcc builds.