branch: let delete_branches warn instead of error on bulk refusal

branch: let delete_branches warn instead of error on bulk refusal

Add a warn_only flag to delete_branches() and check_branch_commit()
so a bulk caller can report not-fully-merged branches as one-line
warnings and continue, instead of erroring with the four-line "use
'git branch -D'" advice that the standalone "git branch -d" path
emits.  Default callers pass 0 and are unaffected.

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