Index.Stage: do not stage ignored files

Stage() now respects ignore files by default, similar to git.git.
Users accustomed to the old behavior may use the new StageOptions
class (in particular the IncludeIgnored bool) to override this
behavior.
Update binaries to 69db893

https://github.com/libgit2/libgit2/compare/091165c...69db893
Enforce Clone() test coverage

Fix #701
Ensure Tags can be created in detached Head state

Fix #791
Minor fix
Remove trailing whitespaces
Drop tests that rely on the global configuration

We cannot make correct assumptions regarding the fact that the
environment that host the test runner:

 - Contains a global git config file
 - Grants the test runner process with enough rights to write to this file
 - That this file already contains an expected entry

Given those constraints, those tests are removed.
Release LibGit2Sharp v0.19
Enforce Index.RetrieveStatus() test coverage