contrib/debian/compat

destutil: choose non-closed branch head at first (BC)

Before this patch, destupdate() returns the tipmost (descendant)
branch head regardless of closed or not. But updating to closed branch
head isn't reasonable for ordinary workflow, because:

  - "hg heads" doesn't show closed heads (= updated parent itself) by
    default

  - subsequent committing on it re-opens closed branch

    even if inactivation of closed head is needed, update destination
    isn't it, because it should be merged into to another branch in
    such case.

This patch chooses non-closed descendant branch head as default update
destination at first. If all descendant branch heads are closed,
destupdate() returns the tipmost closed branch head.

For simplicity, this patch chooses adding _destupdatebranchfallback()
instead largely changing _destupdatebranch().

This patch changes not only normal lookup code path, but also the "no
default branch" code path, for consistency.

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