`sl --version` breaks when building Sapling from an old Sapling

[Max Coplan]

steps to reproduce:

1. get an old sl
   • verify it's old by running any sl command and make sure you get hint[old-version]: WARNING! ...
2. build Sapling (make oss)
3. ./sl --version

Expected:

$ sl --version
Sapling <date-and-stuff>

Actual:

$ sl --version
Saping 4.4.2

Probable cause:

The way setup.py gets the version number is by first finding the path to the vc exe and then running the sl log ... (or git show ...). findhg interprets the hint[old-version] as an error and aborts.

The fix is simply to add hint[old-version] to filterhgerr.

[Max Coplan]

I'm working on the fix rn