Refactor commit signature parser (#30228)

To make it more flexible and support SSH signature.

The existing tests are not changed, there are also tests covering
`parseTagRef` which also calls `parsePayloadSignature` now. Add some new
tests to `Test_parseTagData`

modules/git/commit.go

@@ -26,14 +26,14 @@ type Commit struct {
 	Author        *Signature
 	Committer     *Signature
 	CommitMessage string
-	Signature     *CommitGPGSignature
+	Signature     *CommitSignature
 
 	Parents        []ObjectID // ID strings
 	submoduleCache *ObjectCache
 }
 
-// CommitGPGSignature represents a git commit signature part.
-type CommitGPGSignature struct {
+// CommitSignature represents a git commit signature part.
+type CommitSignature struct {
 	Signature string
 	Payload   string // TODO check if can be reconstruct from the rest of commit information to not have duplicate data
 }