branch: prepare delete_branches for a bulk caller

branch: prepare delete_branches for a bulk caller

Add no_head_fallback and dry_run flags to delete_branches() so a
bulk caller (the upcoming --prune-merged) can ask strictly about
merged-into-upstream without a silent fallback to HEAD, and
rehearse deletions with the same "Would delete branch ..." wording
as the live run. Existing callers pass 0 for both and keep current
behavior.

When no_head_fallback is set, head_rev stays NULL through to
branch_merged(), whose "merged to X but not yet merged to HEAD"
reminder otherwise compares against HEAD. For the bulk caller
every candidate is known to have an upstream, so HEAD is
irrelevant. Guard the block on head_rev so the NULL case skips
it instead of treating "NULL != reference_rev" as "diverges from
HEAD" and emitting a spurious warning.

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