revlog: deal with special "postfix" explicitely
revlog usually use a straight forward '.i' and '.d' naming except for two cases
"in-transaction" changelog, and censoring. Our goal is to let the revlog code
deal with the internal of the file naming itself. To do so, we need to start
dealing with these postfix explicitly.
Differential Revision: https://phab.mercurial-scm.org/D10571