compat: make obshistory._getdifflines compatible with mercurial 4.1 On Mercurial version 4.1 and lower, patch.diff seems to returns an empty list when there is no change. Mercurial 4.2 returns None so we add a if for this case.

obsfate: add a new obsfate template The obsfate template display for each obsolete changeset a line summarizing what changed between the changeset and its successors. This dict is computed in obshistory._preparesuccessorset. It uses obshistory.FORMATSSETSFUNCTIONS which is a list of function that individually compute a part of each dict. You can override fields or add new ones by adding your own function in this list. The format of obsfate is computed in templatekw.obsfatedefaulttempl and can be wrapped if necessary, the code is not quite extendable for the moment but can be refactored later.