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_reader.go

@@ -99,7 +99,7 @@ readLoop:
 		}
 	}
 	commit.CommitMessage = messageSB.String()
-	commit.Signature = &CommitGPGSignature{
+	commit.Signature = &CommitSignature{
 		Signature: signatureSB.String(),
 		Payload:   payloadSB.String(),
 	}