cherry-pick/revert: transparently refresh index
A stat-dirty index is not a detail that ought to concern the operator
of porcelain such as "git cherry-pick".
Without this change, a cherry-pick after copying a worktree with rsync
errors out with a misleading message.
$ git cherry-pick build/top
error: Your local changes to 'file.h' would be overwritten by merge. Aborting.
Please, commit your changes or stash them before you can merge.
Noticed-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>