Model.internalScanFolderSubs: Scan only requested subs.
Model.internalScanFolderSubs: Scan only requested subs.

This reverts the change introduced in 9b9fe0d Reduce scanning effort.
That commit caused us to automatically ignore the basename of the
specified subs and instead scan closest known root folder. For
example, in a folder that looks like:

Sync/
├── 00
│   ├── one
│   ├── three
│   └── two
├── 01
│   ├── one
│   ├── three
│   └── two
├── 02
│   ├── one
│   ├── three
│   └── two
└── one

calling '/rest/db/scan?folder=default&sub=01' called filepath.Walk on
the whole Sync/ folder instead of just the desired subfolder. This
contradicts the scan behavior promised by the documentation.

This is related to #2151.

kluppy/syncthing,letiemble/syncthing,xduugu/syncthing,alex2108/syncthing,ProactiveServices/syncthing,norgeous/syncthing,TheBigBear/syncthing,toddleish/syncthing,AudriusButkevicius/syncthing,nrm21/syncthing,syncthing/syncthing,wweich/syncthing,syncthing/syncthing,norgeous/syncthing,canton7/syncthing,lkwg82/syncthing,le9i0nx/syncthing,nrm21/syncthing,uok/syncthing,cherylquirion/syncthing,Kudalufi/syncthing,toddleish/syncthing,le9i0nx/syncthing,Kudalufi/syncthing,alex2108/syncthing,rumpelsepp/syncthing,ProactiveServices/syncthing,wweich/syncthing,rumpelsepp/syncthing,kamadak/syncthing,cherylquirion/syncthing,rumpelsepp/syncthing,imsodin/syncthing,kralo/syncthing,jryans/syncthing,letiemble/syncthing,norgeous/syncthing,TheBigBear/syncthing,kc1212/syncthing,TheBigBear/syncthing,brgmnn/syncthing,cherylquirion/syncthing,uok/syncthing,xduugu/syncthing,rumpelsepp/syncthing,imsodin/syncthing,syncthing/syncthing,brgmnn/syncthing,plouj/syncthing,brgmnn/syncthing,jryans/syncthing,norgeous/syncthing,kluppy/syncthing,xduugu/syncthing,canton7/syncthing,kamadak/syncthing,kluppy/syncthing,uok/syncthing,le9i0nx/syncthing,syncthing/syncthing,imsodin/syncthing,cherylquirion/syncthing,uok/syncthing,kralo/syncthing,brgmnn/syncthing,kralo/syncthing,letiemble/syncthing,plouj/syncthing,ProactiveServices/syncthing,kc1212/syncthing,lkwg82/syncthing,plouj/syncthing,canton7/syncthing,le9i0nx/syncthing,Kudalufi/syncthing,wweich/syncthing,kralo/syncthing,syncthing/syncthing,lkwg82/syncthing,TheBigBear/syncthing,toddleish/syncthing,wweich/syncthing,ProactiveServices/syncthing,kc1212/syncthing,nrm21/syncthing,kamadak/syncthing,xduugu/syncthing,imsodin/syncthing,AudriusButkevicius/syncthing,toddleish/syncthing,AudriusButkevicius/syncthing,syncthing/syncthing,plouj/syncthing,AudriusButkevicius/syncthing,Kudalufi/syncthing,kamadak/syncthing,alex2108/syncthing,alex2108/syncthing,lkwg82/syncthing,canton7/syncthing,letiemble/syncthing,jryans/syncthing,kluppy/syncthing,imsodin/syncthing,nrm21/syncthing,kc1212/syncthing,jryans/syncthing