Merge branch 'gr/rebase-i-drop-warn' into maint

Merge branch 'gr/rebase-i-drop-warn' into maint

"git rebase -i" had a minor regression recently, which stopped
considering a line that begins with an indented '#' in its insn
sheet not a comment, which is now fixed.

* gr/rebase-i-drop-warn:
  rebase-i: loosen over-eager check_bad_cmd check
  rebase-i: explicitly accept tab as separator in commands