Make sure checkout doesn't report conflicts on ignored paths

In a situation where a certain path was ignored but a working tree file
with this path existed jgit didn't allow to checkout a branch which
didn't ignore this path but contained different content. JGit considered
this to be a checkout conflict to prevent overwriting the file in the
working tree and raised an error. This commit fixes this by ensuring
that ignored dirty working tree files don't lead to a checkout conflict.

Bug: 450169
Change-Id: I90288d314ffac73c24a9c70a5181f8243bd4679a
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>