.gitignore: Readd `/tmp/`
.gitignore: Readd `/tmp/`

It is produced during `./x.py test`
graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust