Fixes#2151.
Since Walk.walkAndHashFiles ignores .stfolder and .stignore, they will
never be found by fs.Get(protocol.LocalDeviceID, sub) in
Model.internalScanFolder. As a result, when asked to scan those subs
we end up scanning the whole folder.
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.
