hgrc: search XDG_CONFIG_HOME on mac
Searching for hgrc was special cased not to look through ~/.config/hg on Mac,
but that’s unnecessary: Macs support it as do other unix based systems. There
are plenty tools that use it there, e.g. git, and people expect it to work, e.g.
"https://stackoverflow.com/questions/72499837/mercurial-on-macos-doesnt-read-config-hg-hgrc".
Initial code introduced in 354020079723.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).