为什么git checkout会为一些未更改的文件打印“M”?

事情就这样发生了:

$ git branch
* master
...lots of other branches...

$ git checkout -tb feat/bandwidthprelude
M   MANIFEST.in
M   lint.sh
M   setup.py
M   ...some other source files...
Branch 'feat/bandwidthprelude' set up to track local branch 'master'.
Switched to a new branch 'feat/bandwidthprelude'

$ git status
On branch feat/bandwidthprelude
Your branch is up to date with 'master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)

    ...[redacted]...

nothing added to commit but untracked files present (use "git add" to track)

git checkout 切换分支时,将打印 M 当文件有本地修改时。但在这种情况下,根据 git status 这些文件没有本地修改 ,尽管它们是我最近编辑和提交的文件)。我确认我最后的编辑没有丢失。 怎么搞的?

git checkout -tb <branchname>