Add --splice support to merge command

[qu3vipon]

This fixes: #1712

Description

I noticed that no one was actively working on this issue, so I went ahead and implemented support for the --splice option in the merge command.

This change allows merge to operate with non-head revisions by disabling the head-only validation, consistent with the behavior of --splice in revision.

Key points included in this PR:

• Adds --splice support to alembic merge, allowing non-head revisions to be used as merge bases.
• Updates the help text to reflect the new behavior.
• Adds test coverage to validate both the failure case (splice=False) and the successful merge case (splice=True) using non-head revisions.

Let me know if further adjustments or additional tests would be helpful.

Checklist

This pull request is:

• [ ] A documentation / typographical error fix
  • Good to go, no issue or tests are needed
• [x] A short code fix
  • please include the issue number, and create an issue if none exists, which must include a complete example of the issue. one line code fixes without an issue and demonstration will not be accepted.
  • Please include: Fixes: #<issue number> in the commit message
  • please include tests. one line code fixes without tests will not be accepted.
• [ ] A new feature implementation
  • please include the issue number, and create an issue if none exists, which must include a complete example of how the feature would look.
  • Please include: Fixes: #<issue number> in the commit message
  • please include tests.

Have a nice day!

[sqla-tester]

New Gerrit review created for change 52bf8a6ca4ad881597a6a4dd15eefa1433a4a259: https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/6496