rebase: simplify documentation about --keep Also include a warning about bookmarks

rebase: simplify documentation about selecting commits to rebase

rebase: simplify documentation about public commits add reference to graft

verify: clean up weird error/warning lists Nested functions in Python are not able to assign to variables in the outer scope without something like the list trick because assignments refer to the inner scope. So, we formerly used a list to give an object to assign into. Now that error and warning are object members, the [0] hack is no longer needed.

diff: clarify comparison as first parent

histedit: add progress support

verify: remove unreachable code to reraise KeyboardInterrupt KeyboardInterrupt should never be caught as it doesn't inherit Exception in Python 2.5 or later. And if it was, "interrupted" would be printed twice. https://docs.python.org/2.7/library/exceptions.html#exception-hierarchy

merge with stable

verify: move exc() function onto class This is part of an effort to make verify more modular so extensions can hook into it.

verify: move err() to be a class function This is part of an effort to make it easier for extensions to hook into verify.