upstream 与 ahead/behind 的计算

  $ git config --get-regexp "^branch\." ; git branch -vv | sed -E 's/ [0-9a-f]{7} / <sha> /'
  branch.main.remote origin
  branch.main.merge refs/heads/main
  * main <sha> [origin/main] m1

  $ git status -sb | head -1; echo "(fetch 之前:不知道远程多了 3 个)"
  ## main...origin/main [ahead 2]
  (fetch 之前:不知道远程多了 3 个)
  $ git fetch -q; git status -sb | head -1; git status | sed -n 2,3p; git branch -vv | sed -E 's/ [0-9a-f]{7} / <sha> /'
  ## main...origin/main [ahead 2, behind 3]
  Your branch and 'origin/main' have diverged,
  and have 2 and 3 different commits each, respectively.
  * main <sha> [origin/main: ahead 2, behind 3] A 本地 2

  $ git log --oneline origin/main..main | sed -E "s/^[0-9a-f]+ //"; echo "-- 以上是领先的提交(只在本地)"; git log --oneline main..origin/main | sed -E "s/^[0-9a-f]+ //"; echo "-- 以上是落后的提交(只在远程)"
  A 本地 2
  A 本地 1
  -- 以上是领先的提交(只在本地)
  B 远程 3
  B 远程 2
  B 远程 1
  -- 以上是落后的提交(只在远程)
  $ git rev-list --left-right --count main...origin/main; echo "(左边 = main 独有,右边 = origin/main 独有)"; git merge-base main origin/main | cut -c1-7 | sed "s/.*/merge-base = <sha>/"; git log -1 --format=%s $(git merge-base main origin/main)
  2	3
  (左边 = main 独有,右边 = origin/main 独有)
  merge-base = <sha>
  m1
  不同的写法等价:
  $ git rev-list --count origin/main..main; git rev-list --count main..origin/main; git rev-list --count ^origin/main main; git rev-list --count main --not origin/main
  2
  3
  2
  2

  $ git rev-parse --abbrev-ref @{u}; git rev-parse --abbrev-ref @{upstream}; git rev-parse --abbrev-ref main@{u}; git rev-parse --abbrev-ref @{push}
  origin/main
  origin/main
  origin/main
  origin/main
  $ git remote add fork /work/l29remote.git; git fetch -q fork; git config remote.pushDefault fork; git rev-parse --abbrev-ref @{push}; echo "-- 换成 push.default=current:"; git -c push.default=current rev-parse --abbrev-ref @{push}; echo "-- @{u} 不受影响:"; git rev-parse --abbrev-ref @{u}
  -- 换成 push.default=current:
  fork/main
  -- @{u} 不受影响:
  origin/main
  fatal: cannot resolve 'simple' push to a single destination
  $ git config --unset remote.pushDefault; git config branch.main.pushRemote fork; git -c push.default=current rev-parse --abbrev-ref @{push}; git config --unset branch.main.pushRemote; git remote remove fork
  fork/main

  $ git switch -q -c plain; git config --get branch.plain.merge || echo "plain: 没有 upstream"
  plain: 没有 upstream
  $ git switch -q -c from-main main; git config --get branch.from-main.merge || echo "from-main(起点是本地分支): 没有 upstream"
  from-main(起点是本地分支): 没有 upstream
  $ git switch -q -c topic origin/topic 2>&1; git config --get branch.topic.remote; git config --get branch.topic.merge
  origin
  refs/heads/topic
  $ git switch -q plain; git push -q -u origin plain 2>&1; git config --get branch.plain.remote; git config --get branch.plain.merge
  origin
  refs/heads/plain
  $ git switch -q from-main; git branch -q --set-upstream-to=origin/main; git rev-parse --abbrev-ref from-main@{u}
  origin/main
  $ git config --get branch.autoSetupMerge || echo "branch.autoSetupMerge 未设置(默认 true:只有起点是 remote-tracking 分支才设置)"
  branch.autoSetupMerge 未设置(默认 true:只有起点是 remote-tracking 分支才设置)

  $ git switch -q topic; git fetch -q --prune; git branch -vv | grep topic | sed -E 's/ [0-9a-f]{7} / <sha> /'
  * topic     <sha> [origin/topic: gone] B 远程 3
  $ git status | sed -n 1,3p
  On branch topic
  Your branch is based on 'origin/topic', but the upstream is gone.
    (use "git branch --unset-upstream" to fixup)