lib/fs: Properly handle case insensitive systems (fixes #1787, fixes #2739, fixes #5708)

With this change we emulate a case sensitive filesystem on top of
insensitive filesystems. This means we correctly pick up case-only renames
and throw a case conflict error when there would be multiple files differing
only in case.

This safety check has a small performance hit (about 20% more filesystem
operations when scanning for changes). The new advanced folder option
`caseSensitiveFS` can be used to disable the safety checks, retaining the
previous behavior on systems known to be fully case sensitive.

Co-authored-by: Jakob Borg <jakob@kastelo.net>

lib/osutil/osutil.go

@@ -131,8 +131,10 @@ func copyFileContents(method fs.CopyRangeMethod, srcFs, dstFs fs.Filesystem, src
 }
 
 func IsDeleted(ffs fs.Filesystem, name string) bool {
-	if _, err := ffs.Lstat(name); fs.IsNotExist(err) {
-		return true
+	if _, err := ffs.Lstat(name); err != nil {
+		if fs.IsNotExist(err) || fs.IsErrCaseConflict(err) {
+			return true
+		}
 	}
 	switch TraversesSymlink(ffs, filepath.Dir(name)).(type) {
 	case *NotADirectoryError, *TraversesSymlinkError: