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>