gitk: Allow safely calling nukefile from a run queue handler

gitk: Allow safely calling nukefile from a run queue handler

Originally dorunq assumed that the queue entry remained first
in the queue after the script eval, and blindly removed it.
However, if the handler calls nukefile, it may not be the
case anymore, and a random queue entry gets dropped instead.

This makes dorunq remove the entry before calling the
script, and adds a global variable to allow other functions
to determine if they are called from within a dorunq handler.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>