Add support for reverting multiple commits at once.

This modifies the Revert Commit menu item in the History view such that
multiple selected commits may be reverted at once, instead of only a
single commit.

JGit-Dependency: I5de3a9a2a4c276e60af732e9c507cbbdfd1a4652
Change-Id: I42cce1cecfb6f871d5187960925bde79603c3a3e
Signed-off-by: Maik Schreiber <blizzy@blizzy.de>