Confirmed that JGit 3.5.2 still prunes more than CliGit

Prune during fetch is currently disabled for JGit because JGit behaves
differently than command line git when pruning during fetch.  The unit
test test_fetch_with_prune() in GitAPITestCase shows the difference.
Merge commit 'fe7f56021994cfde453a7b30091a61df86557a8e' into HEAD