git am --skip: clean the index while preserving local changes

git am --skip: clean the index while preserving local changes

In 3-way merge, "am" will let the index with unmerged path waiting
for us to resolve conflicts and continue. But if we want to --skip
instead, "am" refuses to continue because of the dirty index.

With this patch, "am" will clean the index without touching files
locally modified, before continuing.

Signed-off-by: Olivier Marin <dkr@freesurf.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>