Problem:
When vim._watch.watch() is used to watch a single file, libuv returns
the basename as the filename argument in the callback. The code joins
this with the watched path, producing a nonsensical path like
"/path/to/file.lua/file.lua", which causes ENOTDIR errors on
subsequent fs_stat calls.
Solution:
Check whether the watched path is a directory before joining the
filename. When watching a file, ignore the filename from libuv and
use the watched path directly.
(cherry picked from commit d9d8c660fd)
03815969c4