libgit2/libgit2sharp

Update libgit2 to ef5a385

URLs without a path are perfectly valid; the path is implicitly `/`.
libgit2 was incorrectly rejecting these paths, and LibGit2Sharp was
incorrectly testing this bad behavior.  Remove this test now that
libgit2 has fixed this issue.

Add support for adding and clearing multi-valued configuration

Co-Authored-By: Brandon Ording <bording@gmail.com>

This should fix travis failures in CI

Implement value adding by exposing the underlying `set_multivar`,
which supports only string values in the underlying libgit2, so no
other typed overloads are provided at this point.

The counterpart for deleting keys exposes the underlying `delete_multivar`.
No regex-based overload is exposed for consistency with the existing `Set<T>`
overloads which don't expose it either.

Also exposed the boolean return value from the `Unset` calls which
is already present in the Proxy API.

Fixes #1719.

Fix

Linux: support broader range of distros/architectures by trying to load packed native libraries.

…ad packed native libraries.

LibGit2Sharp includes a number of native libraries that are built on different OSes.
These native libraries have a dependency on OpenSSL and the system c-library.

On x64, in case the Linux flavor is not known, a fallback is performed to the 'linux-x64' native library.
This library is built with a dependency on OpenSSL 1.0.
OpenSSL 1.0 is deprecated by OpenSSL 1.1, so on more recent version of Linux, the 'linux-x64' library fails to load.

On arm64, native libraries are currently included for debian.* (OpenSSL 1.1) and ubuntu.18.04 (OpenSSL 1.0).
Loading on other distros will fail.

In both cases LibGit2Sharp is probably including a library that works, but the default resolution logic
is not able to find it.

By using the 'NativeLibrary' class (.NET Core 3.0) we can extend the resolution logic, and try to load
the other native libraries that are packed with LibGit2Sharp.

Update libgit2 to v0.28.3

 Housekeeping and cleanup updates

Add support for Alpine 3.9

Fix typos in 'Optimizing unit testing'

"Insure" relates to having insurance. "ensure" means "to make sure of"
"One-thousand" shouldn't be hyphenated. It should only be hyphenated in the case of a compound modifier on the word "thousand" as in "forty-five thousand".

Update libgit2 to v0.28.1

Update libgit2 to v0.28.0-pre1

libgit2 has updated some function signatures for consistency,
including the `giterr` family of functions which is now `git_error`.