rules: document the hash when importing from upstream branch
The resulting patch now has a header looking like this:
```
GIT update of https://sourceware.org/git/glibc.git/release/2.41/master from glibc-2.41 to 1502c248d58cb99a203731707987a4342926e830
```
This makes it easier to figure out if a specific patch is already
present or not.