During valgrind runs, we try to swap out the OpenSSL allocators for our
own.  This allows us to avoid some unnecessary warnings about usage.
Unfortunately, many builds of OpenSSL do not allow you to swap
allocators; for example FIPS builds and the builds running in CentOS.

Try to swap the allocators, but do not fail when they cannot be
customized.

sha1dc: remove conditional for <sys/types.h>

ci: add centos 7 and centos 8

ci: update nightly to use source path

http: don't require a password

All the other workflows jobs check out into the `source` directory; do this in the documentation build job as well, for consistency across jobs.

Respect the force flag on refspecs in git_remote_fetch

docker: remove the entrypoint

amiga: use ';' as path list separator on AmigaOS

Omitting an entrypoint.sh to configure the container and instead
depending on docker primitives allows us to be more portable.  (If a
distribution uses a different mechanism for adding users, we need not
have multiple entrypoint.sh files or invariants within it; instead we
can configure that in the dockerfile itself along with all the other
distribution specific components.)

Like on Windows ':' is used for volume names in absolute paths.

Attempt authentication when a username is presented but a password is
not; this can happen in particular when users are doing token
authentication and specifying the token in the URL itself.  For example,
`https://token@host/` is a valid URI and should be treated as a username
of `token` with an empty password.

tests: reset odb backend priority

Fix one memory leak in master

Introduce GIT_OPT_SET_ODB_LOOSE_PRIORITY and GIT_OPT_SET_ODB_PACKED_PRIORITY
to allow overriding the default priority values for the default ODB
backends. Libgit2 has historically assumed that most objects for long-
running operations will be packed, therefore GIT_LOOSE_PRIORITY is
set to 1 by default, and GIT_PACKED_PRIORITY to 2.
When a client allows libgit2 to set the default backends, they can
specify an override for the two priority values in order to change
the order in which each ODB backend is accessed.

There was one test that wasn't correctly disposing of the repository.

Document `GIT_STATUS_OPT_INCLUDE_UNREADABLE`, and some minor cleanups.