t6500(mingw): use the Windows PID of the shell

t6500(mingw): use the Windows PID of the shell

In Git for Windows, we use the MSYS2 Bash which inherits a non-standard
PID model from Cygwin's POSIX emulation layer: every MSYS2 process has a
regular Windows PID, and in addition it has an MSYS2 PID (which
corresponds to a shadow process that emulates Unix-style signal
handling).

With the upgrade to the MSYS2 runtime v3.x, this shadow process cannot
be accessed via `OpenProcess()` any longer, and therefore t6500 thought
incorrectly that the process referenced in `gc.pid` (which is not
actually a real `gc` process in this context, but the current shell) no
longer exists.

Let's fix this by making sure that the Windows PID is written into
`gc.pid` in this test script so that `git.exe` is able to understand
that that process does indeed still exist.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>