abort: added support for histedit
This patch adds the support for histedit in hg abort plan.
As seperate hgaborthistedit() function is created to handle
independent calls for abortion of histedit. This function is
then registered as abortfunc for state detection API.
hg abort in case of histedit also supports history-editing-backup
Results are shown as tests.
Differential Revision: https://phab.mercurial-scm.org/D6582