When the 'Reset first' option is selected for a job, the command run is:

repo forall -c "git reset --hard"

This fails on a Windows slave due to the double quotes. Example output from the Jenkins log is:

12:46:06 [Source] $ repo.cmd forall -c "git reset --hard"
12:46:07 error: unknown option `hard git'

This also inteferes with automatic cleanup if build modifies some committed source and the initial 'repo sync -d -c' cause repo-plugin to try a hard reset. The output is the same.