RFC: SHA256 proof of concept

meta: update version number to v1.6.0-alpha

v1.5.0

Fix creation of branches and tags with invalid names

pack: don't pretend we support pack files v3

zlib: update bundled zlib to v1.2.12

Fixes for CVE 2022-29187

slide_hash knowingly reads (possibly) uninitialised memory, see comment
lower down about prev[n] potentially being garbage. In this case, the
result is never used - so we don't care about MSAN complaining about
this read.

By adding the no_sanitize("memory") attribute, clients of zlib won't
see this (unnecessary) error when building and running with
MemorySanitizer. An alternative approach is for clients to build zlib
with -fsanitize-ignorelist=... where the ignorelist contains something
like 'fun:slide_hash'. But that's more work and needs to be redone
for any and all CI systems running a given project with MSAN. Adding
this annotation to zlib's sources is overall more convenient - but
also won't affect non-MSAN builds.

This specific issue was found while running git's test suite, but has
also been reported by other clients, see e.g. #518.

The `crc32_combine_gen64` missed a prototype in our define path.
Add one.

In the ownership checks implemented for CVE-2022-24765, we disallowed
users to access their own repositories when running with `sudo`.

Examine the `SUDO_UID` environment variable and allow users running
with `sudo`. This matches git's behavior.

To match git's behavior with CVE 2022-29187, validate not only the
working directory, but also the gitdir and gitlink (if it exists). This
a follow up to CVE-2022-24765 that was fixed earlier.

Fix erroneously lax configuration ownership checks

Pack files v3 are introduced in the SHA256 hash transition document
https://github.com/git/git/blob/master/Documentation/technical/hash-function-transition.txt

Obviously we do not support these yet. Stop pretending that we do.

Allow users in the administrator group to use git configs that are owned
by administrators.

Allow the user ownership to match if the file is owned by the admin
group and the user is in the admin group, even if the current process is
not running as administrator directly.

Refactor the file ownership checks so that callers can provide discrete
information about the ownership expectations to a single function.

The file ownership concepts can reflect the actual file ownership, they
are not necessarily limited to mocking the interface. Rename them so
that they can be more broadly applicable.

Ensure that we test opening a bare repository with odd permissions.

This reverts commit cdff2f02.

This change erroneously allowed system users to own a worktree; this
should only be allowed when the current user is in the Administrator
group on Windows as well.

cmake: drop posix dependency from pcre* detection

since f585b129 neither PCRE nor PCRE2 backends rely on POSIX regex, so
reflect it in library detection logic

fix interactive rebase detect.