Right now, the function `git_repository_foreach_head` will always
iterate over all HEADs of the main repository and its worktrees. In some
cases, it might be required to skip either of those, though. Add a flag
in preparation for the following commit that enables this behaviour.

Libraries should use assert(3P) only very scarcely. First, we usually
shouldn't cause the caller of our library to abort in case where the
assert fails. Second, if code is compiled with -DNDEBUG, then the assert
will not be included at all.

In our `git_branch_is_checked_out` function, we have an assert that
verifies that the given reference parameter is non-NULL and in fact a
branch. While the first check is fine, the second is not. E.g. when
compiled with -DNDEBUG, we'd proceed and treat the given reference as a
branch in all cases.

Fix the issue by instead treating a non-branch reference as not being
checked out. This is the obvious solution, as references other than
branches cannot be directly checked out.

We currently do not have any tests at all for the
`git_branch_is_checked_out` function. Add some basic ones.

When adding a new worktree, we only verify that an optionally given
reference is valid half-way through the function. At this point, some
data structures have already been created on-disk. If we bail out due to
an invalid reference, these will be left behind and need to be manually
cleaned up by the user.

Improve the situation by moving the reference checks to the function's
preamble. Like this, we error out as early as possible and will not
leave behind any files.

ci: skip ssh tests on macOS nightly

Like 811c1c0f, disable the SSH tests on
macOS until we can resolve the newly introduced infrastructure issues.

CI build fixups

SSH tests on macOS have begun failing for an unknown reason after an
infrastructure upgrade to macOS 10.13.6.  Disable those tests
temporarily, until we can resolve it.

Subtle changes in the host OS can have impacts in the CI system that
may be hard to debug.  We previously showed the results of `uname` which
can be difficult to interpret.  Provide more information where
available.

The URL was incorrect for the nightly badge image; it was erroneously
showing the master branch continuous integration build badge.

Include a build badge for `maint/v0.28` builds.

v0.28 rc1

Docs

The mbedtls library uses a callback mechanism to allow downstream users
to plug in their own receive and send functions. We implement `bio_read`
and `bio_write` functions, which simply wrap the `git_stream_read` and
`git_stream_write` functions, respectively.

The problem arises due to the return value of the callback functions:
mbedtls expects us to return an `int` containing the actual number of
bytes that were read or written. But this is in fact completely
misdesigned, as callers are allowed to pass in a buffer with length
`SIZE_MAX`. We thus may be unable to represent the number of bytes
written via the return value.

Fix this by only ever reading or writing at most `INT_MAX` bytes.

The mbedtls stream implementation makes use of some global variables
which are not marked as `static`, even though they're only used in this
compilation unit. Fix this and remove a duplicate declaration.

Our `openssl_write` function calls `SSL_write` by passing in both `data`
and `len` arguments directly. Thing is, our `len` parameter is of type
`size_t` and theirs is of type `int`. We thus need to clamp our length
to be at most `INT_MAX`.

Now that the function `git_stream__write_full` exists and callers of
`git_stream_write` have been adjusted, we can lift logic for short
writes out of the stream implementations. Instead, this is now handled
either by `git_stream__write_full` or by callers of `git_stream_write`
directly.

Similar to the write(3) function, implementations of `git_stream_write`
do not guarantee that all bytes are written. Instead, they return the
number of bytes that actually have been written, which may be smaller
than the total number of bytes. Furthermore, due to an interface design
issue, we cannot ever write more than `SSIZE_MAX` bytes at once, as
otherwise we cannot represent the number of bytes written to the caller.

Unfortunately, no caller of `git_stream_write` ever checks the return
value, except to verify that no error occurred. Due to this, they are
susceptible to the case where only partial data has been written.

Fix this by introducing a new function `git_stream__write_full`. In
contrast to `git_stream_write`, it will always return either success or
failure, without returning the number of bytes written. Thus, it is able
to write all `SIZE_MAX` bytes and loop around `git_stream_write` until
all data has been written. Adjust all callers except the BIO callbacks
in our mbedtls and OpenSSL streams, which already do the right thing and
require the amount of bytes written.

The callback functions that implement the `git_stream` structure are
only used inside of their respective implementation files, but they are
not marked as `static`. Fix this.

Documentation fixes

ci: add an individual coverity pipeline

Coverity is back but it's only read-only!  Agh.  Just allow it to fail
and not impact the overall job run.

ci: run docurium to create documentation

ci: return coverity to the nightlies

Clean up some warnings

Nightlies: use `latest` docker images

Run docurium as part of the build.  The goal of this is to be able to
evaluate the documentation in a given pull request; as such, this does
not implement any sort of deployment pipeline.

This will allow us to download a snapshot of the documentation from the
CI build and evaluate the docs for a particular pull request; before
it's been merged.

Validate that the return value of the read is not less than INT_MAX,
then cast.

Index entries are 32 bit unsigned ints, not `size_t`s.