git-cvsexportcommit: Fix calling Perl's rel2abs() on MSYS

git-cvsexportcommit: Fix calling Perl's rel2abs() on MSYS

Due to MSYS path mangling GIT_DIR contains a Windows-style path when
checked inside a Perl script even if GIT_DIR was previously set to an
MSYS-style path in a shell script. So explicitly convert to an MSYS-style
path before calling Perl's rel2abs() to make it work.

This fix was inspired by a very similar patch in WebKit:

http://trac.webkit.org/changeset/76255/trunk/Tools/Scripts/commit-log-editor

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Tested-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>