Fix upgrading changed repository files

DataObj::EPM::enable() would throw an error if a repository file already
existed and differed to its checksum.

Correct behaviour:
DataObj::EPM moves the modified file to .epmsave
Screen::EPMC recognises the .epmsave and replaces the new version with
.epmsave, saving the new version as .epmnew

Work-around:

Before upgrading, disable the package, upgrade and then re-enable it.

Reported by Samuel Pearce @ Open (CORE)