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

It was unused and only defined in posix.h
It did not exist in its Windows equivalent.

strcasecmp is a posix function, testing it doesn't make sense.
Functions that needs unsigned compare should use git__strcasecmp()

cli: add `index-pack` command

ci: allow workflows to push changes

Our workflows push documentation changes; ensure that they are allowed
to do so.

Fix 'path cannot exist in repository' during diff for in-memory repository

ci: allow workflows to read and write packages

Git blame buffer gives the wrong result in many cases where there are…

cmake: rename FindIconv to avoid collision with cmake

Our CI workflows consume and will automatically generate their build
containers. Ensure that they can do so.

cmake now includes `FindIconv`. Rename ours to avoid any confusion.

actions: set permissions

Several compatibility fixes

We now allow leading and trailing dots in username and email addresses.
Test that we do so.

clone: don't mix up "http://url" with "http:/url" when figuring out if we should do a local clone

process: test /usr/bin/false on BSDs