github不小心同步覆盖了本地文件

我在两个电脑上修改一个项目，A修改文件a，B修改文件b。然后我干了下面这些事，按时间顺序。。。

1. A上 git commit
2. A上 git push
3. B上 git commit
4. B上 git pull
   然后。。。B上修改的b就被覆盖了。。。

而且git log已经找不到第3步的commit了。

解决方案：
执行git reflog

502dd0f HEAD@{0}: pull --progress --rebase --prune origin master
147b3b5 HEAD@{1}: commit: commit-mark
502dd0f HEAD@{2}: rebase finished: returning to refs/heads/master

然后

git reset --hard 502dd0f
git cherry-pick 147b3b5

就可恢复原来B上的本地文件， 然后push到服务器端：

git push origin master

但出现：

! [rejected] master -> master (non-fast-forward)
error: failed to push some refs to ‘https://github.com/GitUsername/GitProgramName.git‘
hint: Updates were rejected because a pushed branch tip is behind its remote
hint: counterpart. Check out this branch and integrate the remote changes
hint: (e.g. ‘git pull ...‘) before pushing again.
hint: See the ‘Note about fast-forwards‘ in ‘git push --help‘ for details.

可见，线下线上文件出现了冲突。
解决方法：

git pull
如果有冲突，解决冲突
git push

但是git pull出现问题：

You are not currently on a branch. Please specify which
branch you want to merge with. See git-pull(1) for details.

git pull <remote> <branch>

解决方法：切换到master，再执行git push

git checkout -b temp
git checkout master