git: support separate arg for `--config-env`'s value
While not documented as such, many of the top-level options like
`--git-dir` and `--work-tree` support two syntaxes: they accept both an
equals sign between option and its value, and they do support option and
value as two separate arguments. The recently added `--config-env`
option only supports the syntax with an equals sign.
Mitigate this inconsistency by accepting both syntaxes and add tests to
verify both work.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>