Add a performance optimized variant of the ANY_DIFF filter

Add a performance optimized variant of the ANY_DIFF filter

If a treewalk walks also over index and the workingtree then the
IndexDiffFilter filter can be used which works much faster then
the semantically equivalent ANY_DIFF filter. This is because this
filter can better avoid computing SHA-1 ids over the content of
working-tree files which is very costly.

This fix will significantly improve the performance of e.g.
EGit's commit dialog.

Change-Id: I2a51816f4ed9df2900c6307a54cd09f50004266f
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Signed-off-by: Philipp Thun <philipp.thun@sap.com>