format-patch: pick up correct branch name from symbolic ref

format-patch: pick up correct branch name from symbolic ref

find_branch_name() assumes to take refs/heads/<branch>. But we also
have symbolic refs, such as HEAD, that can point to a valid branch in
refs/heads and do not follow refs/heads/<branch> syntax. Remove the
assumption and apply normal ref resolution. After all it would be
confusing if rev machinery resolves a ref in one way and
find_branch_name() another.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>