Line counters: do not wrap when over 999 on mobile

Summary:

Before	After

Closes T15615

Test Plan:
Create a very long Diff with soooo much lines (e.g. 1000+).
Reduce the window and check that line numbers do not wrap anymore.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15615

Differential Revision: https://we.phorge.it/D25407