Refactor TemporaryFileSwap.__init__ for clarity

This changes the mkstemp call TemporaryFileSwap uses to atomically
create (and thus reserve) the temporary file, so that it passes
only a filename (i.e., basename) prefix as `prefix`, and passes all
other path components (i.e., the directory) as `dir`. (If the
original path has no directory, then `dir` is "" as before.)

This makes the mkstemp call *slightly* more idiomatic. This also
makes it clearer, as it is no longer using `prefix` for something
that feels like it should be possible to pass as a Path object.

Although mkstemp does not accept a Path as `prefix`, it does (as
expected) accept one as `dir`. However, to keep the code simple,
I'm passing str for both. The os.path.split function accepts both
str and Path (since Python 3.6), and returns str objects, which are
now used for the `dir` and `prefix` arguments to mkstemp.

For unusual cases, this may technically not be a refactoring. For
example, a file_path of "a/b//c" will be split into "a/b" and "c".
If the automatically generated temporary file suffix is "xyz", then
that results in a tmp_file_path of "a/b/cxyz" where "a/b//cxyz"
would have been used before. The tmp_file_path attribute of a
TemporaryFileSwap object is public (and used in filesystem calls).

However, no guarantee has ever been given that the temporary file
path have the original path as an exact string prefix. I believe
the slightly weaker relationship I expressed in the recently
introduced test_temporary_file_swap -- another file in the same
directory, named with the original filename with more characters,
consisting of equivalent path components in the same order -- has
always been the intended one.

Note that this slight possible variation does not apply to the
file_path attribute. That attribute is always kept exactly as it
was, both in its type and its value, and it always used unmodified
in calls that access the filesystem.

git/index/util.py

@@ -41,7 +41,8 @@ class TemporaryFileSwap:
 
     def __init__(self, file_path: PathLike) -> None:
         self.file_path = file_path
-        fd, self.tmp_file_path = tempfile.mkstemp(prefix=str(self.file_path), dir="")
+        dirname, basename = osp.split(file_path)
+        fd, self.tmp_file_path = tempfile.mkstemp(prefix=basename, dir=dirname)
         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)