rust-core: extracted a revlog submodule
This moves fundamental definitions from the top of the crate
to the newly created `revlog` submodule and reexports them for
easy compatibility.
As we are about to add new features to this crate, we felt it
will improve clarity, and moreso if `ancestors` and `dagops`
would become submodules of `revlog`.
Differential Revision: https://phab.mercurial-scm.org/D7782
$ hg init
$ touch a
$ unset HGUSER
$ echo "[ui]" >> .hg/hgrc
$ echo "username= foo" >> .hg/hgrc
$ echo " bar1" >> .hg/hgrc
$ hg ci -Am m
adding a
abort: username 'foo\nbar1' contains a newline
[255]
$ rm .hg/hgrc
$ HGUSER=`(echo foo; echo bar2)` hg ci -Am m
adding a
abort: username 'foo\nbar2' contains a newline
[255]
$ hg ci -Am m -u "`(echo foo; echo bar3)`"
adding a
transaction abort!
rollback completed
abort: username 'foo\nbar3' contains a newline!
[255]