rebase -ir should warn (or error out) when commits were produced but not included in the final commit history

[dscho]

It is easy to do this:

pick something

reset somewhere else
pick something different
label this-is-my-branch

reset onto
merge this-is-my-branch

Obviously, the first pick was not included, by mistake.

So every time reset is executed, it should record the HEAD's hash in a growing list, and verify in the end that they are reachable (possibly using the refs/rewritten/* refs with name-rev to display nicer names).

This feature is similar in spirit to the warning when calling git checkout <branch> on a detached HEAD, where the user is told about the commits that would now be lost.

Migrated from https://bugs.chromium.org/p/git/issues/detail?id=32