Previously, users could skip certain tests when running the CI script
(`SKIP_ONLINE_TESTS=1`). Provide a mechanism for users to opt-in to
only certain tests (`SKIP_TESTS=1 SKIP_ONLINE_TESTS=0`).

README: replace gmaster with GitButler

master is no more; GitButler is a new client using libgit2.

Fixes #6689

cli: add `index-pack` command

ci: allow workflows to push changes

Our workflows push documentation changes; ensure that they are allowed
to do so.

Fix 'path cannot exist in repository' during diff for in-memory repository

ci: allow workflows to read and write packages

Git blame buffer gives the wrong result in many cases where there are…

cmake: rename FindIconv to avoid collision with cmake

Our CI workflows consume and will automatically generate their build
containers. Ensure that they can do so.

cmake now includes `FindIconv`. Rename ours to avoid any confusion.

actions: set permissions

Several compatibility fixes

We now allow leading and trailing dots in username and email addresses.
Test that we do so.

clone: don't mix up "http://url" with "http:/url" when figuring out if we should do a local clone

process: test /usr/bin/false on BSDs

Support setting oid type for in-memory repositories

Pass a pointer to the quote counts so that we can increment it.

Ensure that we declare variables at the top of the block for broad
compatibility with old compilers.

push: set generic error in push_negotiation cb

release: add a compatibility label

Add a compatibility label to the release.yml to describe things that are for improved cross-platform compatibility.

When a user returns `-1` in a `push_negotiation` callback, we set the
error to whatever's hanging out in the buffer, probably something about
a missing configuration entry.

Clear the error buffer before invoking the callback, so that if a user
does not set an error message in their callback that we can detect. If
there is no error but `-1` is returned, set a generic error message.

fetch: enable deepening/shortening shallow clones

util: suppress some uninitialized variable warnings

Avoid macro redefinition of ENABLE_INTSAFE_SIGNED_FUNCTIONS