pkg/cover: factor out common code into combinePrefix
pkg/cover: factor out common code into combinePrefix

Combining of prefixes is now repeated in 2 places.
Factor it out into a common function.

google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller