Pure go clone+commit on Windows drops execution bit

[puerco]

There seems to be a strange bug running sourcetool on windows that drops the unix execution bit from scripts and commits the changes.

The bit change can be because of limitations in the windows filesystem, but I suspect the changes being committed points to a bug in go-git.

[puerco]

Ref: protobom/protobom@dbbb719
protobom/protobom#355