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>

git-cvsexportcommit.perl

@@ -30,6 +30,13 @@
 		chomp($gd);
 		$ENV{GIT_DIR} = $gd;
 	}
+
+	# On MSYS, convert a Windows-style path to an MSYS-style path
+	# so that rel2abs() below works correctly.
+	if ($^O eq 'msys') {
+		$ENV{GIT_DIR} =~ s#^([[:alpha:]]):/#/$1/#;
+	}
+
 	# Make sure GIT_DIR is absolute
 	$ENV{GIT_DIR} = File::Spec->rel2abs($ENV{GIT_DIR});
 }