dirstate: restore original estimation and update comment
The former comment didn't reflect the content of the dirstate entries,
the two nodes are a fixed header in the file and not per-entry. Split
the documented entry into the path part and the fixed header. The
heuristic itself is still valid, e.g. for the NetBSD src tree a maximum
path size of 142 and an average of 49, resulting in 66 bytes per entry
on average.
Differential Revision: https://phab.mercurial-scm.org/D8850