Support authentication in push example

Yet more CI improvements

Using modern rsa (sha2-256 or sha2-512) with libssh2 is complicated and
depends on numerous factors for support. Just use ecdsa, which is
supported by both libssh2 v1.11.0 and modern OpenSSH (which we use for
our server in CI).

Let's use the latest & greatest for building our sanitizer / fuzzing
builds.

httpclient: clear client->parser.data after use

ntlmclient: update to latest upstream ntlmclient

git_diff_find_similar doesn't always remove unmodified deltas

Some static code analysis complains that we're putting a stack variable
into client->parser.data (which persists past the function calls). Clear
that on function exit to avoid confusion.

Fix some bugs caught by UBscan

`git_commit_create_v` takes _commits_ not _commit ids_. Fix the test to
call the API correctly.

Passing a function pointer with different parameters is UB, even if
those params are (void *). Use our separate `_cb` functions that will do
the correct casting.

zlib: upgrade bundled zlib to v1.3

examples: use unsigned int for bitfields

Bypass shallow clone support for in-memory repositories

blame example: Fix support for line range in CLI

Minor bug fixes

ci: improvements to prepare for Cygwin support

Ensure that the main builds are all identical in nightly. Nightly should
only add new, specialized builds.

Setup scripts can change BUILD_WORKSPACE, for instance, if they use a
different format for paths, they can replace with `cygpath` variants.

Allow the build matrix to specify its shell, in case we provide one in
the setup (eg, Cygwin).

Always checkout out shell scripts with Unix-style line endings. mingw
doesn't mind, but cygwin struggles without CRLF.

`opts` may be null; check before dereferencing.

`GIT_ASSERT` may be a macro for `assert` (when `GIT_ASSERT_HARD` is
defined), which may differ in debug builds. Pull the assignment out of
the assertion.

If we're reading an on-disk packfile, ensure that we close the file
descriptor on error.

CI fixes

tests: remove test for strcasecmp

process: don't try to close the status

Previously, users could skip certain tests when running the CI script
(`SKIP_ONLINE_TESTS=1`). Provide a mechanism for users to opt-in to
only certain tests (`SKIP_TESTS=1 SKIP_ONLINE_TESTS=0`).

README: replace gmaster with GitButler

`process->status` is a status value; we were mistakenly trying to close
it as a file descriptor, as if it were the `status` self-pipe that we open
during process creation.

Instead, don't try to close it, as it's not a file descriptor.

master is no more; GitButler is a new client using libgit2.

Fixes #6689