Prepare EGit for the new State MERGING_RESOLVED

Prepare EGit for the new State MERGING_RESOLVED

JGit will report a new repository state MERGING_RESOLVED. Currently EGit
doesn't support committing in this state. Missing is the correct handling
of the content of .git/MERGE_MESSAGE and .git/MERGE_HEAD. When committing
in this state these contents have to be used and after a successful commit
these files have to be deleted.

This fix adds a special check for this new state and disallows committing
in this state.

This change depends on JGit change I0a885e2fe8c85049fb23722351ab89cf2c81a431.
This change will break the build of EGit if this JGit change is not merged.
On the other hand: As soon as the JGit change has been merged we'll have to
merge this change soon.

Change-Id: Ie28c2b43d168b8d6a2873bf9d09a1be9cc8bbf15
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>