Implement support for revision specification syntax

[GlebChili]

@KalleOlaviNiemitalo noted in dotnet/Nerdbank.GitVersioning#702 that Nerdbank.GitVersioning managed git implementation does not supported @ syntax for specifying revisions as documanted in https://git-scm.com/docs/git-rev-parse#Documentation/git-rev-parse.txt-emem. In fact, the major part of the revision specification syntax is not supported (as described in https://git-scm.com/docs/git-rev-parse#_specifying_revisions).

ManagedGitLib should support such revision specification syntax.