$SourceFilename for post_or pre_commands has wrong filename on Windows

source_post_commands, source_pre_commands, target_post_commands and target_pre_commands can use $SourceFilename which is substitited with the filename of the transferred source file.

On Windows it get a wrong name.
For example
source_dir = C:\TEMP\jade
file_spec= \.txt$
-> $SourceFilename = C:\TEMP\jade/C:\TEMP\jade\1.txt