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