branchmap: add some clarifications and clean up flow - Remove indentation where it is not needed. - Swap the subset test branches to follow along logically and put the 'empty' case last. Differential Revision: https://phab.mercurial-scm.org/D5637

branchmap: updating triggers a write Rather than separate updating and writing, create a subclass that doesn't write on update. This minimises chances we forget to write out updates somewhere. This also makes refactoring and improving the branchmap functionality easier. Differential Revision: https://phab.mercurial-scm.org/D5636

branchmap: make branchcache responsible for reading Encapsulate reading in a classmethod, to make it clear what kind of object is being handled. This is part of a stack of refactoring changes to help performance improvements down the line. Differential Revision: https://phab.mercurial-scm.org/D5635

attr: make some docstrings raw strings This avoids a SyntaxWarning in Python 3.8 due to invalid \ escapes. Differential Revision: https://phab.mercurial-scm.org/D5817

revset: leverage getintrange() helper in relation-subscript operation (API) Now a range expression is parsed by a relation function itself since the upper layer have no knowledge about the default first/last bounds.

revset: allow to parse single integer as a range Even though this wouldn't be any useful in followlines(), it helps to unify range processing.

revset: extract a helper to parse integer range It's getting common. As a first step, this patch adds getintrange() and makes followlines() use it. I wanted to unify the error messages to make the function interface simple, but I failed to phrase it briefly.