Patch Set 1:

Shouldn't it in the end not be a command in our org.eclipse.jgit.api package? I am sure that people outside jgit would like to have a git-show implementation which is re-useable and has a a clean, type-safe interface. Code in the pgm package is not reuseable (it's not visible e.g. to egit), accpets all parameters as Strings and dumps return values to stdout.
It's not much - just moving that part of the code which does the real work into a comamnd.