Fix a PHP 8.1 deprecated use of preg_match with a NULL argument

Summary:
These calls are preventing users to browse subversion/mercurial repositories in PHP 8.1+.
Indeed, a similar bug affecting git repositories was already addressed in another commit
(rP6b8ec50148909938850b5acfd11725ae23a8e31b). This commit harmonize both DiffusionSvnRequest
and DiffusionMercurialRequest with DiffusionGitRequest

Fix T15607

Test Plan:

• Sign in
• Open a diffusion SVN/Mercurial repository
• You should not get a RuntimeException concerning this preg_match call

Reviewers: O1 Blessed Committers, Sten, valerio.bozzolan

Reviewed By: O1 Blessed Committers, Sten, valerio.bozzolan

Subscribers: Sten, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15607

Differential Revision: https://we.phorge.it/D25397