tests: Add `--rhg` and `--with-rhg=<path>` options for `run-tests.py`
They are mostly equivalent to the corresponding `chg` options.
For now, many tests are still failing in this configuration.
It is *not* run on CI.
Differential Revision: https://phab.mercurial-scm.org/D10095
#require execbit
$ hg init
$ echo a > a
$ hg ci -Am'not executable'
adding a
$ chmod +x a
$ hg ci -m'executable'
$ hg id
79abf14474dc tip
Make sure we notice the change of mode if the cached size == -1:
$ hg rm a
$ hg revert -r 0 a
$ hg debugstate
n 0 -1 unset a
$ hg status
M a
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg id
d69afc33ff8a
$ test -x a && echo executable -- bad || echo not executable -- good
not executable -- good