Updates vendor/code.gitea.io/git (#6286)

Gopkg.lock

 
 [[projects]]
   branch = "master"
-  digest = "1:0f0ada42a7b1bd64794bf8fea917c2cd626d6b0539173e3704cd764b93eb5312"
+  digest = "1:e1fa64238b0a2dbf1edf98c4af8d1b8cb65179e286d7f28006b50fa9f508ee9d"
   name = "code.gitea.io/git"
   packages = ["."]
   pruneopts = "NUT"
-  revision = "8983773ac6fef49203e7ee8cdbfde3e118bc3421"
+  revision = "74d7c14dd4a3ed9c5def0dc3c1aeede399ddc5c5"
 
 [[projects]]
   branch = "master"
     "gopkg.in/ini.v1",
     "gopkg.in/ldap.v3",
     "gopkg.in/macaron.v1",
+    "gopkg.in/src-d/go-git.v4",
+    "gopkg.in/src-d/go-git.v4/config",
+    "gopkg.in/src-d/go-git.v4/plumbing",
     "gopkg.in/testfixtures.v2",
     "strk.kbt.io/projects/go/libravatar",
   ]

vendor/code.gitea.io/git/repo_commit.go

 				sig, err := newGPGSignatureFromCommitline(data, (nextline+1)+sigindex, true)
 				if err == nil && sig != nil {
 					// remove signature from commit message
-					cm = cm[:sigindex-1]
+					if sigindex == 0 {
+						cm = ""
+					} else {
+						cm = cm[:sigindex-1]
+					}
 					commit.Signature = sig
 				}
 			}
 func (repo *Repository) GetCommit(commitID string) (*Commit, error) {
 	if len(commitID) != 40 {
 		var err error
-		commitID, err = NewCommand("rev-parse", commitID).RunInDir(repo.Path)
+		actualCommitID, err := NewCommand("rev-parse", commitID).RunInDir(repo.Path)
 		if err != nil {
 			if strings.Contains(err.Error(), "unknown revision or path") {
 				return nil, ErrNotExist{commitID, ""}
 			}
 			return nil, err
 		}
+		commitID = actualCommitID
 	}
 	id, err := NewIDFromString(commitID)
 	if err != nil {