remote: write camel-cased *.pushRemote on rename

remote: write camel-cased *.pushRemote on rename

When a remote is renamed don't change the canonical "*.pushRemote"
form to "*.pushremote". Fixes and tests for a minor bug in
923d4a5ca4f (remote rename/remove: handle branch.<name>.pushRemote
config values, 2020-01-27). See the preceding commit for why this does
& doesn't matter.

While we're at it let's also test that we handle the "*.pushDefault"
key correctly. The code to handle that was added in
b3fd6cbf294 (remote rename/remove: gently handle remote.pushDefault
config, 2020-02-01) and does the right thing, but nothing tested that
we wrote out the canonical camel-cased form.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>