Stub out the `true` flag for `ghe-config`
Stub out the `true` flag for `ghe-config`

The new MySQL backup code uses `ghe-config --true ...`.  The mock in
`test/bin/ghe-config` prints out a lot of error messages when it gets that
flag, which makes debugging test failures harder.

This commit fixes the mock so it implements `ghe-config --true ...`
correctly, the same way recent versions of GHES do.

github/backup-utils,github/backup-utils