templater: remember cache key of evaluated revset
This provides a hint for caching further computation result of the given
revset. See the next patch for example.
fixbundle() {
grep -v 'saving bundle' | grep -v 'saved backup' | \
grep -v added | grep -v adding | \
grep -v "unable to find 'e' for patching" | \
grep -v "e: No such file or directory" | \
cat
}