什么是……在git命令中是什么意思?

它只是指定一个范围。我会用“to”代替它,就像“committa to commitb”一样。

相关的Git手册 man git-diff 说:

git diff [<options>] <commit> <commit> [--] [<path>...]
    This is to view the changes between two arbitrary <commit>.

git diff [<options>] <commit>..<commit> [--] [<path>...]
    This is synonymous to the previous form. If <commit> on one
    side is omitted, it will have the same effect as using HEAD
    instead.