Rely on parent pom for Guava dependency

The guava version must be kept in tight synchronization with the
version used in the parent pom, so there is little value declaring it
here, then manually maintaining its version in sync with the parent.
Rely on git-client-plugin to provide JGit

Avoid tracking the JGit version number, simplify pom management.

Similar technique already used successfully in the git-server plugin.