branch: add --dry-run for --prune-merged

branch: add --dry-run for --prune-merged

With --dry-run, --prune-merged prints the local branches it would
delete -- one "Would delete branch <name>" line per candidate --
and exits without touching any ref.

This is the natural sanity check before letting a broad pattern
like 'origin/*' run for real: the @{push}-vs-@{upstream} and
unmerged filtering still applies, so the dry-run output is
exactly the set that the live run would delete.

--dry-run is only meaningful in combination with --prune-merged
and is rejected otherwise.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>