Introduce MergeFetchHeadNotFoundException

This adds a  strongly typed exception to indicate when a pull is
performed and a reference for the configured merge branch was not fetched.
Update exception message when Pull cannot find ref to merge
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.