rhg: more efficient `HgPath::join` This commit makes `HgPath::join` slightly more efficient by avoiding one copy. It also avoids a particularly inefficient (quadratic) use of `HgPath::join` by using a new mutating function `HgPathBuf::push` instead. The name for `HgPathBuf::push` is chosen by analogy to `PathBuf::push`. Differential Revision: https://phab.mercurial-scm.org/D11721