Leave working dir and index alone after failed (conflicting) push

This leaves the index and working tree in the state that merge-recursive
left it, with unmerged files in different stages, and the non-conflicting
changes in the index.

Signed-off-by: David Kågedal <davidk@lysator.liu.se>
Signed-off-by: Karl Hasselström <kha@treskal.com>