cmd: %~dp0 should expand to the directory containing the batch file, not the current directory

[Dan Kegel]

Fixes http://bugs.winehq.org/show_bug.cgi?id=21382
See the attached test case there, which passes
on winetestbot and on wine after my patch, but fails on unpatched wine.
Makes chromium's build happier.

Also verified with valgrind that no new warnings were introduced.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: dp0.patch
Type: text/x-patch
Size: 2097 bytes
Desc: not available
URL: <http://www.winehq.org/pipermail/wine-patches/attachments/20100202/08e76b96/attachment.bin>