Convert plumbing.ErrObjectNotFound to git.ErrNotExist in getCommit (#10862) (#10868)

Backport #10862

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: guillep2k <18600385+guillep2k@users.noreply.github.com>
Co-authored-by: Antoine GIRARD <sapk@users.noreply.github.com>

modules/git/repo_commit.go

@@ -12,9 +12,9 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/mcuadros/go-version"
 	"github.com/go-git/go-git/v5/plumbing"
 	"github.com/go-git/go-git/v5/plumbing/object"
+	"github.com/mcuadros/go-version"
 )
 
 // GetRefCommitID returns the last commit ID string of given reference (branch or tag).
@@ -94,9 +94,15 @@ func (repo *Repository) getCommit(id SHA1) (*Commit, error) {
 	gogitCommit, err := repo.gogitRepo.CommitObject(id)
 	if err == plumbing.ErrObjectNotFound {
 		tagObject, err = repo.gogitRepo.TagObject(id)
+		if err == plumbing.ErrObjectNotFound {
+			return nil, ErrNotExist{
+				ID: id.String(),
+			}
+		}
 		if err == nil {
 			gogitCommit, err = repo.gogitRepo.CommitObject(tagObject.Target)
 		}
+		// if we get a plumbing.ErrObjectNotFound here then the repository is broken and it should be 500
 	}
 	if err != nil {
 		return nil, err