Add FetchSHA abstraction to LocalRepository

Originally reported in !2693 (comment 435231130)

Fetching a single SHA is done in multiple places across Gitaly and is very verbose and error prone. Having a definitive and easy to use function will improve our code quality. Adding a new method FetchSHA to localrepo could enable this.

Edited Jul 29, 2021 by Sami Hiltunen

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information