when resetting to HEAD~ safe HEAD's message to ring

When `magit-reset', `magit-reset-head' or `magit-reset-soft' are used to
reset to HEAD~, then save the commit message to the message ring used by
`git-commit-mode' using the new function `magit-maybe-save-head-message'.
when rebase stops on a commit select in the status buffer

Add new sentinel `magit-rebase-process-sentinel' and use it for
processes started by `magit-rebase-async'.  After it has run the
standard sentinel it also moves point in the status buffer to the
stopped-at commit.
magit-log-select-mode-map: bind "e" to magit-log-select-pick

To edit a single commit one can first move to that commit in the status
or log buffer and then press "e e e" to edit just that one commit.  If
one doesn't first move to that commit one has to select the commit to
start with in a `magit-log-select-mode' buffer, like

  e e n... C-c C-c e

now it is possible to do this instead

  e e n... e e

which is closer to the recommended way

  n... e e e
magit-insert-rebase-sequence: also insert rebased commits

After the commits yet to be rebased also insert the commits that were
already rebased.  This gives more context and allows quickly splitting
up the stopped at commit by soft resetting to the preceding commit.
Merge branch 'n/rebase-goodies' into next