Match Mavens version ordering when choosing the highes version

[Feuermurmel]

a9153c9 introduced an enhancement which will pick the "newest" version from tags if there are multiple tags on the same commit. Sadly, the algorithm deciding which version is the "newest" one does not implement the same logic used by Maven (See http://www.mojohaus.org/versions-maven-plugin/version-rules.html and http://maven.apache.org/ref/3.0.4/maven-artifact/xref/org/apache/maven/artifact/versioning/DefaultArtifactVersion.html). The following project demonstrates a situation in which the version which is chose is not the one which was intended:

$ cat project/plugins.sbt 
addSbtPlugin("com.typesafe.sbt" % "sbt-git" % "1.0.0")
$ cat build.sbt 
enablePlugins(GitVersioning)
git.gitTagToVersionNumber := { Some(_).filter(_ matches "[0-9]+\\..*") }
$ git tag 1.0
$ git tag 1.0-alpha1

$ sbt version
[...]
[info] 1.0-alpha1

I'm suggesting to use the same logic implemented by Maven.