mingw: fix regression in t1308-config-set

mingw: fix regression in t1308-config-set

When we tried to fix in 58461bd (t1308: do not get fooled by symbolic
links to the source tree, 2016-06-02) an obscure case where the user
cd's into Git's source code via a symbolic link, a regression was
introduced that affects all test runs on Windows.

The original patch introducing the test case in question was careful to
use `$(pwd)` instead of `$PWD`.

This was done to account for the fact that Git's test suite uses shell
scripting even on Windows, where the shell's Unix-y paths are
incompatible with the main Git executable's idea of paths: it only
accepts Windows paths.

It is an awkward but necessary thing, then, to use `$(pwd)` (which gives
us a Windows path) when interacting with the Git executable and `$PWD`
(which gives the shell's idea of the current working directory in Unix-y
form) for shell scripts, including the test suite itself.

Obviously this broke the use case of the Git maintainer when changing
the working directory into Git's source code directory via a symlink,
i.e. when `$(pwd)` does not agree with `$PWD`.

However, we must not fix that use case at the expense of regressing
another use case.

Let's special-case Windows here, even if it is ugly, for lack of a more
elegant solution.

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