Git.pm: Set GIT_WORK_TREE if we set GIT_DIR

Otherwise git will use the current directory as work tree which will
lead to unexpected results if we operate in sub directory of the
work tree.

Signed-off-by: Frank Lichtenheld <flichtenheld@astaro.com>
Acked-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/perl/Git.pm b/perl/Git.pm
index 291ff5b..4313db7 100644
--- a/perl/Git.pm
+++ b/perl/Git.pm
@@ -1280,6 +1280,8 @@
 	my ($self, @args) = @_;
 	if ($self) {
 		$self->repo_path() and $ENV{'GIT_DIR'} = $self->repo_path();
+		$self->repo_path() and $self->wc_path()
+			and $ENV{'GIT_WORK_TREE'} = $self->wc_path();
 		$self->wc_path() and chdir($self->wc_path());
 		$self->wc_subdir() and chdir($self->wc_subdir());
 	}