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`
config option.
Results are shown as tests.
Differential Revision: https://phab.mercurial-scm.org/D6582
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