Consider replacing in-house Git ref sanitizer with git-check-ref-format

[KexyBiscuit]

• The current one:

  buildit/server/src/api.rs

  Line 98 in 7ef76b0

  // sanitize git_branch arg

• The official one: https://www.git-scm.com/docs/git-check-ref-format

This affects branch names with low line _, like AOSC-Dev/aosc-os-abbs#7230.

However, due to #23 (comment), this cannot be directly implemented like #23.