Allow bypassing check for '.keep' file

Deprecation: export the deprecated functions properly

Enable hard deprecation in our builds to ensure that we do not call
deprecated functions internally.

Add a CMake option to enable hard deprecation; the resultant library
will _not_ include any deprecated functions.  This may be useful for
internal CI builds that create libraries that are not shared with
end-users to ensure that we do not use deprecated bits internally.

Although the error functions were deprecated, we did not properly mark
them as deprecated.  We need to include the `deprecated.h` file in order
to ensure that the functions get their export attributes.

Similarly, do not define `GIT_DEPRECATE_HARD` within the library, or
those functions will also not get their export attributes.  Define that
only on the tests and examples.

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.