templatespec: create a factory function for each type there is Most of the arguments to the `templatespec` constructor are mutually exclusive, so each combination creates a different type of templatespec. Let's clarify that by creating factory functions. I've left the callers in `logcmdutil` unchanged for now because they are more complex and `logcmdutil.templatespec()` is slightly higher level in that it is specific to changesets. My larger goal is to add support frozen binaries (specifically PyOxidizer) by adding a specific type of `templatespec` for built-in templates. That will get its own factory function. Differential Revision: https://phab.mercurial-scm.org/D8845