Patch Set 3:

Rebased the change set

>Assuming that bug tracker and git repository reside on the same host is an over-simplification which e.g. fails for Eclipse

This is true, but if repository url is not provided at all, things go even worse. 
Mylyn will use first repository provider which will say that it found something like task id in commit message.

I've made this url resolving optional now, so if nothing matches, the adapter will return null as before.

For bugs.eclipse.org Open Corresponding task works if full bug url is pasted into commit message.

What do you think if repository url will be read from some special config entry? For example 'bugtracker.url'