templatekw: move getrenamedfn() to scmutil (API)

Authored by martinvonz.

Description

templatekw: move getrenamedfn() to scmutil (API)

The function is already used by hg log (for following renames, not
for templates), so it seems it does not belong in templatekw.

Differential Revision: https://phab.mercurial-scm.org/D6021

Details