Since 2185362 (built-in rebase: call `git am` directly, 2019-01-18),
the built-in rebase already uses the built-in `git am` directly.

Now that d03ebd4 (rebase: remove the rebase.useBuiltin setting,
2019-03-18) even removed the scripted rebase, there is no longer any
user of `git-rebase--am.sh`, so let's just remove it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

One test case's title mentioned the then-current implementation detail
that the `--am` backend was implemented in `git-rebase--am.sh`.

This is no longer the case, so let's update the title to reflect the
current reality.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

This went away in 0609b74 (rebase -i: combine rebase--interactive.c
with rebase.c, 2019-04-17).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Update a code comment that referred to those files as if they were still
there.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

The only remaining scripted part of `git rebase` is the
`--preserve-merges` backend. Meaning: there is little reason to keep the
"library of common rebase functions" as a separate file.

While moving the functions to `git-rebase--preserve-merges.sh`, we also
drop the `move_to_original_branch` function that is no longer used.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

We started marking all of those test cases that require `git rebase -p`
to work in the recent past, to allow for skipping them (because the
`--preserve-merges` backend is already deprecated and will be removed in
one of the next major versions, so it is a bit pointless to test its
functionality all the time).

This patch marks two previously forgotten test cases with the `REBASE_P`
prerequisite, to allow skipping them via setting the environment
variable `GIT_TEST_SKIP_REBASE_P`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>