Update libgit2 to build with GIT_TRACE

Build libgit2 with tracing information so that we can provide it to
consumers.
Update libgit2 to e0383fa
Introduce logging

Consumers can register a GlobalSettings.LogConfiguration to receive
logging messages from LibGit2Sharp and trace messages from libgit2.
Simplify Travis install phase.
Fix compareOptions handling in Diff.Compare<T>

Fix #827