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
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