Fix TemporaryFileSwap bug when file_path is a Path

This fixes the regression introduced in 9e86053 (#1770) where the
file_path argument to TemporaryFileSwap.__init__ could no longer be
a Path object.

The change also makes this truer to the code from before #1770,
still without the race condition fixed there, in that str was
called on file_path then as well. However, it is not clear that
this is a good thing, because this is not an idiomatic use of
mkstemp. The reason the `prefix` cannot be a Path is that it is
expected to be a filename prefix, with leading directories given in
the `dir` argument.

git/index/util.py

@@ -41,7 +41,7 @@ class TemporaryFileSwap:
 
     def __init__(self, file_path: PathLike) -> None:
         self.file_path = file_path
-        fd, self.tmp_file_path = tempfile.mkstemp(prefix=self.file_path, dir="")
+        fd, self.tmp_file_path = tempfile.mkstemp(prefix=str(self.file_path), dir="")
         os.close(fd)
         with contextlib.suppress(OSError):  # It may be that the source does not exist.
             os.replace(self.file_path, self.tmp_file_path)