This change adds:
* `tag.yml`: Creates a new tag every time the master or a release branch
is pushed.
* `backport.yml`: Creates a cherry-pick in older release branches to
keep them up to date with little cost.
This change:
* Builds the library with Go 1.14, too.
* Builds the non-legacy tests with Ubuntu Focal (20.04).
* Adds testing for system-wide libraries, both static and dynamic
versions.
* Fixes a typo in the README.
This change:
* Updates the GitHub actions so that they run different commands for the
dynamic and static flavors of libgit2.
* Updates the .travis.yml file so that it does roughly the same as the
GitHub actions.
* Adds the release-* branches to the CI configurations.