Move WinHTTP selection into its own cmake module.

Move SSH selection into its own cmake module.

Move regex selection into its own cmake module.

Move http_parser selection into its own cmake module.

There's no need to add the deprecation at the top-level.  Our tests add
deprecation explicitly.

WinHTTP can now be disabled with `USE_WINHTTP=OFF` instead of
`WINHTTP=OFF` to better support the other cmake semantics.

Threading can now be disabled with `USE_THREADS=OFF` instead of
`THREADSAFE=OFF` to better support the other cmake semantics.

Nanosecond support is the default _if_ we can detect it.  This should be
our default always - like threads - and people can opt out explicitly.

path: refactor utility path functions

Support checking for object existence without refresh

Document that `git_odb` is thread-safe

Commit 4ae41f9c made `git_odb`
race-free, and added internal locking. Update `docs/threading.md`
accordingly, so that APIs built atop libgit2 (e.g. language bindings)
can count on this.

Teach `git_path_is_valid` to respect `core.longpaths`.  Add helper
methods to validate length and set the error message appropriately.

Add a `git_str` based validity check; the existing `git_path_is_valid`
defers to it.

Provide a mechanism for users to limit the number of characters that are
examined; `git_fs_path_str_is_valid` and friends will only examine up to
`str->size` bytes.

`git_fs_path_is_valid` delegates to these new functions by passing
`SIZE_MAX` (instead of doing a `strlen`), which is a sentinel value
meaning "look for a NUL terminator".

Since we're returning a boolean about validation, the name is more
properly "is valid".

Since we're returning a boolean about validation, the name is more
properly "is valid".

Introduce `git_fs_path`, which operates on generic filesystem paths.
`git_path` will be kept for only git-specific path functionality (for
example, checking for `.git` in a path).

ci: only update docs on push

Only update the documentation on a `push`.  We were previously updating the documentation only when not in a `pull_request`, which meant that we would push documentation updates in manual build triggers.

To accommodate less capable compilers.

Co-authored-by: Peter Pettersson <85582231+boretrk@users.noreply.github.com>

Add optional refreshing in the fake backend, and count the number of
refresh calls if enabled.

Looking up a non-existent object currently always invokes
`git_odb_refresh`. If looking up a large batch of objects, many of which
may legitimately not exist, this will repeatedly refresh the ODB to no
avail.

Add a `git_odb_exists_ext` that accepts flags controlling the ODB
lookup, and add a flag to suppress the refresh. This allows the user to
control if and when they refresh (for instance, refreshing once before
starting the batch).

Commit b1a6c316 moved auto-refresh into
the pack backend, and added a comment accordingly. Commit
43820f20 moved auto-refresh back *out*
of backends into the ODB layer, but didn't update the comment.

libgit2_clar is now libgit2_tests

in #6083 the test runner was renamed to libgit2_tests,
but not all references to the old name were updated.
this change changes all of them to use the new name.

cmake: cleanups and consistency

Ensure that we always use lowercase function names, and that we do not
have spaces preceding open parentheses, for consistency.