patchbomb: build patch texts by _getpatchmsgs() Now _getpatchmsgs() knows revision numbers, which allows us to generate flags by applying a template to changectx objects.

patchbomb: drop internal option for pbranch extension (API) I want to move _getpatches() to _getpatchmsgs() to make sure each patch text is tied with the corresponding revision number. This helps adding templater support. IIRC, the pbranch extension doesn't work with the recent Mercurial versions, so the removal of this option wouldn't hurt.

patchbomb: factor out function that builds a prefix string to patch subject I'll add templating support.

formatter: add argument to change output file of non-plain formatter This allows us to build data not written to the console. That would be doable by ui.pushbuffer()/popbuffer(), but changing the file object seems cleaner.