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

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

Add "drop commit-object-name subject" command as another way to
skip replaying of a commit in "rebase -i", and then punish those
who do not use it (and instead just remove the lines) by throwing
a warning.

* gr/rebase-i-drop-warn:
  git rebase -i: add static check for commands and SHA-1
  git rebase -i: warn about removed commits
  git-rebase -i: add command "drop" to remove a commit