replay: offer an option to linearize the commit topology

One of the stated goals of git-replay(1) is to allow implementing the
git-rebase(1) functionality on the server side.

The default mode of git-rebase(1) is to act as if `--no-rebase-merges`
was given. This mode drops merge commits instead of replaying them, and
linearizes the history into a sequence of regular (single-parent)
commits.

Add option `--linearize` to git-replay(1) to do the same. Each replayed
commit is stacked on top of the previously replayed one. When a merge is
encountered, the commits reachable from all of its sides are replayed
into the single line and the merge itself is dropped.

If a ref was pointing to a merge commit, that ref is updated to the
merge's last replayed ancestor.

git-replay(1) accepts multiple branches, for example:

    $ git replay --onto main topic1 topic2

Without `--linearize` this replays 'topic1' and 'topic2' onto 'main'
(keeping shared portions of history shared and divergent parts
divergent) and updates both refs.

Due to current implementation limitations, replaying multiple branches
with `--linearize` is disallowed to avoid concatenating unrelated
histories into a single line. For the same reason disallow the use of
`--contained` with `--linearize`.

Users who want to linearize multiple branches are advised to do this in
separate git-replay(1) invocations. Linearizing multiple branches at
once might be added later.

Note that `--linearize` is not modeled after git-rebase(1)'s
`--rebase-merges[=<mode>]` interface. Recreating merges, by preserving
their topology, is a distinct operation that would be a separate mode.
`--linearize` only drops merges and replays commits linearly. So
git-replay(1) uses its own option rather than reusing that interface.

Based-on-patches-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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