histedit: extract bookmark logic in a dedicated function
This lighten the main function and will help to see future changes to this
bookmark logic.
hide outer repo
$ hg init
$ echo '[Section]' >> $HGRCPATH
$ echo 'KeY = Case Sensitive' >> $HGRCPATH
$ echo 'key = lower case' >> $HGRCPATH
$ hg showconfig Section
Section.KeY=Case Sensitive
Section.key=lower case