doc: merge-tree: improve example script

• Provide a commit message in the example command.

  The command will hang since it is waiting for a commit message on
  stdin.  Which is usable but not straightforward enough since this is
  example code.
• Use `||` directly since that is more straightforward than checking the
  last exit status.

  Also use `echo` and `exit` since `die` is not defined.
• Expose variable declarations.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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