Use the result of reference.Parse when checking for digests.

No need to do a separate string search when reference.Parse has already
determined this.

Changes behavior: non-SHA256 digest formats are now recognized as well.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

Closes: #1112
Approved by: rhatdan
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f