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