Simple function added that check is valid git repository path without initializing it

NullRepositorySafeHandle based proposal
Align Proxy methods name to libgit2 ones
Fix incorrect obsolete message
Linux build: clean directory before build as it may exist already.
Fix .mailmap
Teach Checkout() to cope with revparse syntax
Move BuildSignatureFromGlobalConfiguration() to Configuration
Introduce AttachedHead and teach objects inheritance to LambdaEqualityComparer
Teach Checkout() to append to the reflog

When no user information exists in the configuration, a dummy signature
is created to value the reflog entry.