Merge 'builtin-rebase--am'

This patch teaches the builtin rebase to avoid the scripted --am backend
and call `git format-patch` and `git am` directly.

Meaning: apart from the --merge and the --preserve-merges backends, `git
rebase` is now implemented in pure C, with no need to ask the Unix shell
interpreter for help.

This brings us really close to a fully builtin `git rebase`: the
--preserve-merges mode is about to be deprecated (as soon as the
--rebase-merges mode has proven stable and robust enough), and there are
plans to scrap the `git-rebase--merge` backend in favor of teaching the
interactive rebase enough tricks to run the --merge mode, too.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho