In a PR, the author pushed “.DS_Store” file unintentionally, then he removed ".DS_Store" and edited ".gitignore" file to prevent this happen again.

In this case, when generating webrev, skara would add ".DS_Store" and ".gitignore" both to the working directory and try to push it to the remote repository. This triggers the git error like this

The following paths are ignored by one of your .gitignore files:
.DS_Store
hint: Use -f if you really want to add them.
hint: Turn this message off by running
hint: "git config advice.addIgnoredFile false"

And in that case, the bot will keep pushing and flooding the commit history of the remote repo