branch: add --delete-merged <branch>

branch: add --delete-merged <branch>

    git branch (--delete-merged <branch>)... [<pattern>...]

deletes local branches matching the optional patterns when their
configured upstream matches one of the --delete-merged arguments and
their tip is reachable from that upstream. The work has already landed
on the upstream they track, so the local copy is no longer needed.

The option can be repeated to widen the upstream match. Keeping the
candidate patterns as positional arguments lets users bound the set of
local branches that may be deleted independently of the upstream
selection.

A branch is not deleted when:

  * it is checked out in any worktree
  * its configured upstream ref no longer exists, since a missing
    upstream is not by itself a sign of integration
  * pushing it by name to the remote configured by
    branch.<name>.remote would update its upstream, as determined by
    mapping the branch ref through that remote's fetch refspec. For
    example, a local "main" that tracks "origin/main" is kept even when
    remote.pushDefault names a fork. Right after a pull it merely looks
    fully merged.

A branch whose work is not yet merged into its upstream is silently
skipped, so one unmerged topic does not abort the whole sweep.

A branch that a surviving branch depends on through a chain of local
upstreams is also kept, so no branch is deleted out from under stacked
work. Collect this transitive set without changing the candidate set
during ref iteration: walk upstream chains from surviving branches,
visit each branch at most once, and remove the collected bases only
after the iteration completes. This makes the result independent of
ref iteration order without repeated full scans.

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