rhg: add RootCommand using hg-core FindRoot operation to prepare `hg root`
The println macro is not used to avoid string usage.
Dealing only with bytes allows us to be compatible with any encoding
and not just UTF8.
Later on, format macro will be made to have more readable code.
Differential Revision: https://phab.mercurial-scm.org/D8612
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..