Backports for v1.7

Test proxies specified by both host:port format in configuration
options, environment variables, and `http.proxy` configuration.

The common format for specifying proxy URLs is just 'host:port'. Handle
the common case.

Introduce a url parser that defaults to treating poorly specified URLs
as http URLs. For example: `localhost:8080` is treated as
`http://localhost:8080/` by the http-biased url parsing, instead of a
URL with a scheme `localhost` and a path of `8080`..

Refactor url parsing to simplify the state-passing (introducing a
struct) and add a path parser for future reusability.

We are allowed to call `git_repository__cleanup` multiple times, and this
happens e.g. in rugged if we want to free up resources before GC gets around to
them.

This means that we cannot leave dangling pointers in it, which we were doing
with the grafts. Fix this by setting the pointers to NULL after freeing the resources.

Add a missing include for `git2/oidarray.h` so build doesn't fail on
using `git_oidarray` when using custom transports.

Fixes #6607

- Constant only available in 10.13+, causing build failures for older macOS releases
Fixes: https://github.com/libgit2/libgit2/issues/6606

v1.7: release Kleine Raupe Nimmersatt

deps: update pcre to 8.45

#6576 git_diff_index_to_workdir reverse now loads untracked content

tests: add allocator with limited number of bytes

Fixes #6344: git_branch_move now renames the reflog instead of deleting.

Partial fix for #6532: insert-by-date order.

midx: do not try to look at every object in the index

pack: cast the number of objects to size_t

CMake: Search for ssh2 instead of libssh2.

midx: allow unknown chunk ids in multi-pack index files

docs: fix IRC server from freenode to libera

This commit update the contributing documents with the
new IRC server and remove the old freenode.

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

Fixes "CMake Error: could not resolve ssh2" on Windows-MSVC.

Similar to previous issues around doing way too much verification at loading
time, checking whether the object index chunk is monotonic is better left for
git-fsck instead of every single time we want to look at something in the
repository.

As midx files grow, this starts taking more and more times. As an example, I
went looking for this because it's taking about 1.5s to do a single object
lookup in a repository that's ended up with a 7G multi-pack-index file.

Similar to a previous change where we had to change the casting when loading the
index file, we also need to make sure we don't restrict the numbers to 32-bit
when looking up objects in packfiles.

This was done about three years ago in git itself, but we never got he update in
this library.

These chunks work like extensions where it's fine not to know what one means. We
can skip over it and keep processing the file instead of erroring out.

When a worktree is not prunable, an error message will be set with
information about why. Document that.

PR #5712 predates several refactorings to move the utility code into a
more general purpose codebase. Update to reflect the refactorings.