topic: drop support for accessing csets in branch stack using bxx (issue6119)
When topic extension is enabled and we have some cset whose hash is `b1234`,
topic extension thinks that we are accessing 1234 cset in current branch stack.
However that's not the case generally. Also I am not sure many people use this
bxxx thing.
Since we have a generic sxxx way to access csets, let's drop support for
accessing csets using bxx which leads to bad behavior.
Looking at the tests, we don't show bxxx in hg stack output anymore.
I update the test to use sxxx instead of bxxxx.