Add a merge command to the jgit API

Merges the current head with one other commit.
In this first iteration the merge command supports
only fast forward and already up-to-date.

Change-Id: I0db480f061e01b343570cf7da02cac13a0cbdf8f
Signed-off-by: Stefan Lay <stefan.lay@sap.com>
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>
8 files changed