stg coalesce: Support --file and --save-template

stg coalesce: Support --file and --save-template

--save-template was a bit tricky, because we want that

  * if we reached the stage where the message is needed without
    conflicts, the message should be written and no other side effects
    should occur; but

  * if we run into conflicts before reaching that point, behave just
    as if --save-template was not given.

This makes this script

  stg coalesce --save-template <patches>
  if template was saved:
    let user edit template
    if user didn't abort:
      stg coalesce --file <patches>

equivalent to

  stg coalesce <patches>

with the added benefit that the user can abort the whole thing without
visible side effects.

Signed-off-by: Karl Hasselström <kha@treskal.com>