Bugfix: get mergeRefUrl from source repository, not destination one

Stashbot is currently unable to merge pull requests from a separate fork because the URL used to synthesize the mergeRefUrl parameter is the one belonging to the destination fork (which admittedly is the same as the destination one in most cases).

Please note that this patch can be applied cleanly only after pull request #6 (Feature/ssh), otherwise the checking of the "useSsh" flag (lines 267-270, 272) will cause compilation errors.