verify: rename "hasmanifest" variable for source code readability
Before this patch, there are two ambiguous variables: "havemf" and
"hasmanifest".
"havemf" means whether there are any "manifest" entries.
"hasmanifest" means whether there are any "changelog" entries
referring to "manifest" entry.
This patch renames from "hasmanifest" to "refersmf" to clear
difference from "havemf".
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null