Shortened SHA1 digests in Stash comments.

Soon after we introduced Stashbot at our company, developers started complaining at the "cryptic" comments the bot was adding to their PRs. After a few conversations it turned out that the length of the SHA1 digests in the message is a cognitive barrier for surprisingly many people, experienced developers included -- some of them weren't even aware of the links at the end of the message (!) After applying this patch to reduce the digests in comments to 8 characters, all complains vanished :-)