Fix go-generate for multiple directories

[burik666]

If you change files in multiple directories, go generate will return an error.

named files must all be in one directory; have foo and bar

Issue #76

[dnephin]

Thank you for the fix! I believe this should fix the problem, but now that we have a pipe, I guess this script probably needs

set -eu -o pipefail

To ensure that the exit code is correct. What do you think?

[burik666]

@dnephin, you're right, fixed.