[Bug Report] Fix IsPathInDir

[WithoutPants]

From Discord:

https://github.com/stashapp/stash/blob/develop/pkg/utils/file.go#L275 <-- IsPathInDir returns true when dir is inside pathToCheck and when pathToCheck is inside dir.

eg. IsPathInDir("/media/.previews", "/media") and IsPathInDir("/media","/media/.previews") both return true, the first one shouldn't. .previews is in media, media is not in .previews.

Took me quite a while to work out why my scanning stopped working, because it wont scan /media because due to PR#1118 and the above, it stopped scanning /media, when it should've just stopped scanning /media/.previews

the check inside IsPathInDir should be changed to
if strings.HasPrefix(rel, "..") {