More robust merges

[samoht]

Currently, there are two situations that Irmin do not handler very well:

• trying to merge when no parents in common. This usually do not happen, but why not try to support it (even if it is not very efficient). Also, it might be necessary to handle if we deal with partial history (see Fixes #21). This means adding a merge2 function in user-defined contents.
• trying to merge where multiple common ancestor exists. Currently, Irmin fails (see https://github.com/mirage/irmin/blob/master/lib/core/irminCommit.ml#L180) but we should be able to 3-merge all the pair of parents recursively to get a unique ancestor.

[yansh]

@samoht Strange it still throws the exception for me. Has the irmin package been updated or I need to compile it from the source? sorry, I thought this bug was resolved :) @niksu

[samoht]

Fixed in 0.9.0