t4126: make sure a directory with SP at the end is usable

t4126: make sure a directory with SP at the end is usable

As afb31ad9 (t1010: fix unnoticed failure on Windows, 2021-12-11)
said:

    On Microsoft Windows, a directory name should never end with a period.
    Quoting from Microsoft documentation[1]:

	Do not end a file or directory name with a space or a period.
	Although the underlying file system may support such names, the
	Windows shell and user interface does not.

    [1]: https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file

and the condition addressed by this change is exactly that.  If the
platform is unable to properly create these sample patches about a
file that lives in a directory whose name ends with a SP, there is
no point testing how "git apply" behaves there on the filesystem.

Even though the ultimate purpose of "git apply" is to apply a patch
and to update the filesystem entities, this particular test is
mainly about parsing a patch on a funny pathname correctly, and even
on a system that is incapable of checking out the resulting state
correctly on its filesystem, at least the parsing can and should work
fine.  Rewrite the test to work inside the index without touching the
filesystem.

Helped-by: Jeff King <peff@peff.net>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>