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