CI: Use 'git ls-files' to list all git tracked files

[seisman]

We used the find command to find all files in the current directory and exclude the .git directory. The find command is very long and difficult to understand/maintain.

Instead, we can use git ls-files to list all git-tracked files.

ref: https://stackoverflow.com/a/41085442

Patches #736

[seisman]

/format

[weiji14]

Much more simpler, thanks!