Save errno in handle_alias()

Save errno in handle_alias()

git.c:main() relies on the value of errno being set by the last attempt to
execute the command. However, if something goes awry in handle_alias(),
that assumption is wrong. So restore errno before returning from
handle_alias().

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>