commands: add checks for unfinished operations (
issue3955)
commands checked:
backout
bisect
commit
graft
import
update: clear any clearable unfinished operations (
issue3955)
Unclearable operations will cause an abort.
cmdutil: core functionality to block during multistep commands (
issue3955)
This adds a registration point and check functions that will allow
commands to check if multistep operations like an interrupted graft or
rebase are in progress before proceeding.