replay: make atomic ref updates the default behavior

The git replay command currently outputs update commands that can be
piped to update-ref to achieve a rebase, e.g.

  git replay --onto main topic1..topic2 | git update-ref --stdin

This separation had advantages for three special cases:
  * it made testing easy (when state isn't modified from one step to
    the next, you don't need to make temporary branches or have undo
    commands, or try to track the changes)
  * it provided a natural can-it-rebase-cleanly (and what would it
    rebase to) capability without automatically updating refs, similar
    to a --dry-run
  * it provided a natural low-level tool for the suite of hash-object,
    mktree, commit-tree, mktag, merge-tree, and update-ref, allowing
    users to have another building block for experimentation and making
    new tools

However, it should be noted that all three of these are somewhat
special cases; users, whether on the client or server side, would
almost certainly find it more ergonomic to simply have the updating
of refs be the default.

For server-side operations in particular, the pipeline architecture
creates process coordination overhead. Server implementations that need
to perform rebases atomically must maintain additional code to:

  1. Spawn and manage a pipeline between git-replay and git-update-ref
  2. Coordinate stdout/stderr streams across the pipe boundary
  3. Handle partial failure states if the pipeline breaks mid-execution
  4. Parse and validate the update-ref command output

Change the default behavior to update refs directly, and atomically (at
least to the extent supported by the refs backend in use). This
eliminates the process coordination overhead for the common case.

For users needing the traditional pipeline workflow, add a new
--ref-action=<mode> option that preserves the original behavior:

  git replay --ref-action=print --onto main topic1..topic2 | git update-ref --stdin

The mode can be:
  * update (default): Update refs directly using an atomic transaction
  * print: Output update-ref commands for pipeline use

Test suite changes:

All existing tests that expected command output now use
--ref-action=print to preserve their original behavior. This keeps
the tests valid while allowing them to verify that the pipeline workflow
still works correctly.

New tests were added to verify:
  - Default atomic behavior (no output, refs updated directly)
  - Bare repository support (server-side use case)
  - Equivalence between traditional pipeline and atomic updates
  - Real atomicity using a lock file to verify all-or-nothing guarantee
  - Test isolation using test_when_finished to clean up state
  - Reflog messages include replay mode and target

A following commit will add a replay.refAction configuration
option for users who prefer the traditional pipeline output as their
default behavior.

Helped-by: Elijah Newren <newren@gmail.com>
Helped-by: Patrick Steinhardt <ps@pks.im>
Helped-by: Christian Couder <christian.couder@gmail.com>
Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: Siddharth Asthana <siddharthasthana31@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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