graft: introduce --base option for using custom base revision while merging The graft command usually performs an internal merge of the current parent revision with the graft revision, using p1 of the grafted revision as base for the merge. As a trivial extension of this, we introduce the --base option to allow for using another base revision. This can be used as a building block for grafting and collapsing multiple changesets at once, or for grafting the resulting change from a merge as a single simple change. (This is kind of similar to backout --parent ... only different: this graft base must be an ancestor, but is usually *not* a parent.) This is probably an advanced use case, and we do thus not show it in the non-verbose help.