Fix for https://github.com/ktoso/maven-git-commit-id-plugin/issues/146

@ktoso ,
Well I think this tiny bit of code-change already fixes #146.
In case of detached HEAD-State the JGit-Implementation uses the SHA of the commit. With this PR the same behavoir is implemented for the Native Git-Implementation.

Considered following:
1) Even if the the general Execution-Error (git-Binary couldn't be found), this error will be triggered by the "getCommitId"-Call. 
2) In Terms of Performance we could swap "determineBranchName" and "getCommitId" in the GitDataProvider. When the "getCommitId" will be triggered before the "determineBranchName" we could save the SHA of the current commit inside the Native-Git-Implementation instead of just asking again.

Additional comments are welcome...