Merge branch 'jvanhercke-jgit-default-charset' into master

Merge branch 'jvanhercke-jgit-default-charset' into master

This branch mostly records the commit from PR #1253, so we have it in
the code base. The changes are immediately undone, because the update
to JGit version 4.8.0, as proposed in PR #1252, renders the whole
patching of JGit obsolete.

The update is done now, against reservations in that PR, because we
guess that potential issues have been found and fixed by now. JGit
is updated to the last version on the major version 4 line: 4.11.9.
Also, since upstream JGit is at v6 by now, more updates will have to
happen later on anyways.

To make the protection against illegal character sets complete, an
additional fix for Bugtraq is included, that protects against exceptions
from bugtraq should it encounter an `encoding` that cannot be handled.