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.

common.h: use inline when compiling for C99 and later

Support reading attributes from a specific commit

len, array -> array, len

This change introduces a new API function
`git_graph_reachable_from_any()`, that answers the question whether a
commit is reachable from any of the provided commits through following
parent edges.

This function can take advantage of optimizations provided by the
existence of a `commit-graph` file, since it makes it faster to know
whether, given two commits X and Y, X cannot possibly be an reachable
from Y.

Part of: #5757

commit-graph: Introduce `git_commit_list_generation_cmp`

This change makes calculations of merge-bases a bit faster when there
are complex graphs and the commit times cause visiting nodes multiple
times. This is done by visiting the nodes in the graph in reverse
generation order when the generation number is available instead of
commit timestamp. If the generation number is missing in any pair of
commits, it can safely fall back to the old heuristic with no negative
side-effects.

Part of: #5757