merge: disallow merge abort in case of an unfinished operation (issue6160)
This patch disallows `hg merge --abort` in case an operation of higher
precedence i.e unshelve, rebase, histedit are in unfinished states.
This is done so as to avoid partial abort of these operations in case
merge abort is called at an interrupted step.
The patch adds a `cmdutil.getunfinishedstate` function which checks
for operations under progress and returns a `statecheck` object for it.
Differential Revision: https://phab.mercurial-scm.org/D6607