write-only inner merge support

[max630]

User must be able to

1. specify merge, specifying as the other parent created during current rebase
2. continue pick sequence from some arbitrary commit, rather than last created. Commit have created during the rebase should be able to be used.

In original rebase, this could be performed with 'exec' keyword, and commands "git branch" (to create reference for later use), and "git reset". 'exec' could be also introduced for rebase2. Alternative ideas:

• introduce notion "former commit ". Very nice idea, but does not work for merges. And theoretically a commit can be used several times, creating a ambiguity, but this at least can be catched at todo parsing
• add to commands 'pick' and 'edit' key '--onto', which points to the previous commit
• provide dedicated commands 'branch' and 'reset'

UP:

• other ideas - use ranges for pick, add possibility to make several heads
• so, final syntax of pick is: pick [--onto base1,base2...baseN] from1,from2...fromN..to1,to2..toN it should apply all patches which have at lease one ancestor from toX, and is an ancestor to at least one of fromX. They are applied over --onto (number of baseX should exactly match number of fromX) (TODO: think what could omitting --onto mean, maybe it should be there always)
• with addition of notion for "commit which has been hash", this could enable relatively nice syntax for inner merges

[max630]

The most rough idea is just introduce a new commands to be used instead of exec ..., for reseting, creating references and mentioning them in merge and other commands. For example:

• reset _ref_
• branch _name_. Would create in a specially prefixed ref. No check for existence - they intended to be temporary.
• a prefix for picking the temporary references. Should not be mistaken for already used branch. Since syntax for branches is quite limited I can choose from quite wide list. Note that they probably cannot be used for exec