Mark JGit plugin as 0.6

Our project plan calls for us to build 0.6 as the next version,
but I forgot to also update the MANIFEST.MF for the plugin when
I edited the Maven pom.

Change-Id: Ic1a6c64374a4384a65a3dd0306adddfc73adac52
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>