This change removes the `LIB_INSTALL_DIR` and `INCLUDE_INSTALL_DIR` from
the `script/build-libgit2.sh` script, since they are now unneeded and
just print a warning if they are used.
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.
lhchavez
Carlos Martín Nieto