ci: make grouping setup more generic

ci: make grouping setup more generic

Make the grouping setup more generic by always calling `begin_group ()`
and `end_group ()` regardless of whether we have stubbed those functions
or not. This ensures we can more readily add support for additional CI
platforms.

Furthermore, the `group ()` function is made generic so that it is the
same for both GitHub Actions and for other platforms. There is a
semantic conflict here though: GitHub Actions used to call `set +x` in
`group ()` whereas the non-GitHub case unconditionally uses `set -x`.
The latter would get overriden if we kept the `set +x` in the generic
version of `group ()`. To resolve this conflict, we simply drop the `set
+x` in the generic variant of this function. As `begin_group ()` calls
`set -x` anyway this is not much of a change though, as the only
commands that aren't printed anymore now are the ones between the
beginning of `group ()` and the end of `begin_group ()`.

Last, this commit changes `end_group ()` to also accept a parameter that
indicates _which_ group should end. This will be required by a later
commit that introduces support for GitLab CI.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>