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.
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