Add mergetool support to the classic StGit infrastructure

Since Git already has a tool for interactively solving conflicts which
is highly customisable, there is no need to duplicate this feature via
the i3merge and i2merge configuration options. The user-visible change
is that now mergetool is invoked rather than the previously customised
interactive merging tool.

The stgit.keeporig option is no longer available to be more consistent
with the Git behaviour.

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>
Acked-by: Karl Hasselström <kha@treskal.com>