feature: provide a topgit recognized bash entry to filter branch when do tg update

[LinboLen]

• topgit working on top on original git. lots of conflict need to be solved
• provide bash entry to do some file transform to reduce conflict. lots of refactor can be done by script.
• if not use bash entry. can provide a yaml defined script. just like travis ci grammer. it can be simple provide bash execute tag. such as when do update. when do add depend.

the name of file maybe .topgit.yml

[mackyle]

Perhaps you could provide an example of what you're trying to accomplish?

[LinboLen]

I can't remember that the scenario i need it.