Fixes multiple KVersionControlPlugin::fileName() calls on entering or updating directory.

Summary:
BUG: 415698
FIXED-IN: 20.04

On each VCS plugin creation corresponding file name is saved (cached) so when we search which VCS plugin is appropriate for current directory we don't need to call KVersionControlPlugin::fileName() again.

Reviewers: Dolphin, meven, elvisangelaccio, ngraham

Reviewed By: Dolphin, meven, ngraham

Subscribers: kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D26962