pathutil: document that dirs map type implies manifest/dirstate processing

git: pass a list to pathutil.dirs to indicate that it is a manifest The python implementation of pathutil.dirs just uses a for loop which happens to work the same on both dicts and lists. The rust implementation actually figures out which of the two types it is, and directs the execution to either dirstate or manifest processing.

git: implement basic bookmark activation This is very limited, but it allows 'hg update foo' when already on foo. The caching is based on bmstore's caching.