am: tighten a conditional that checks for $dotest

In preparation for a later patch that creates $dotest/autostash in
git-rebase.sh before anything else happens, don't assume that the
presence of a $dotest directory implies the existence of the
$dotest/next and $dotest/last files.  Look for them explicitly.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
rebase -i: don't error out if $state_dir already exists

In preparation for a later patch that will create $state_dir/autostash
in git-rebase.sh before anything else can happen, change a `mkdir
$state_dir` call to `mkdir -p $state_dir`.  The change is safe,
because this is not a test to detect an in-progress rebase (that is
already done much earlier in git-rebase.sh).

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
rebase: prepare to do generic housekeeping

On successful completion of a rebase in git-rebase--$backend.sh, the
$backend script cleans up on its own and exits.  The cleanup routine
is however, independent of the $backend, and each $backend script
unnecessarily duplicates this work:

    rm -rf "$state_dir"
    git gc --auto

Prepare git-rebase.sh for later patches that return control from each
$backend script back to us, for performing this generic cleanup
routine.  The code that this patch adds is currently unreachable, and
will only start to be used when git-rebase--$backend.sh scripts are
taught to return control in later patches.

Another advantage is that git-rebase.sh can implement a generic
finish_rebase() to possibly do additional tasks in addition to the
cleanup.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
am: return control to caller, for housekeeping

We only need to do these two tasks

    git gc --auto
    rm -fr "$dotest"

ourselves if the script was invoked as a standalone program; when
invoked with --rebasing (from git-rebase--am.sh), cascade control back
to the ultimate caller git-rebase.sh to do this for us.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
rebase -i: return control to caller, for housekeeping

Return control to the caller git-rebase.sh to get these two tasks

    rm -fr "$dotest"
    git gc --auto

done by it.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
rebase --merge: return control to caller, for housekeeping

Return control to the caller git-rebase.sh to get these two tasks

    rm -fr "$dotest"
    git gc --auto

done by it.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
rebase: implement --[no-]autostash and rebase.autostash

This new feature allows a rebase to be executed on a dirty worktree or
index.  It works by creating a temporary "dangling merge commit" out
of the worktree and index changes (via 'git stash create'), and
automatically applying it after a successful rebase or abort.

rebase stores the SHA-1 hex of the temporary merge commit, along with
the rest of the rebase state, in either
.git/{rebase-merge,rebase-apply}/autostash depending on the kind of
rebase.  Since $state_dir is automatically removed at the end of a
successful rebase or abort, so is the autostash.

The advantage of this approach is that we do not affect the normal
stash's reflogs, making the autostash invisible to the end-user.  This
means that you can use 'git stash' during a rebase as usual.

When the autostash application results in a conflict, we push
$state_dir/autostash onto the normal stash and remove $state_dir
ending the rebase.  The user can inspect the stash, and pop or drop at
any time.

Most significantly, this feature means that a caller like pull (with
pull.rebase set to true) can easily be patched to remove the
require_clean_work_tree restriction.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>