git syntax requires uppercase HEAD

[nacnudus]

Perhaps something has changed since the code was written, or perhaps it never worked on linux, but either way this currently doesn't work.

https://github.com/alphagov/tech-docs-template/blame/b37e89431e84c6e23ce6941e8315b2bbcdd12c1a/Thorfile#L127

Results in a file .template_version

---
:remote: git@github.com:alphagov/tech-docs-template.git
:revision: |-
  fatal: ambiguous argument 'head': unknown revision or path not in the working tree.
  Use '--' to separate paths from revisions, like this:
  'git <command> [<revision>...] -- [<file>...]'
  head

A correct file would be like

---
:remote: https://github.com/alphagov/tech-docs-template.git
:revision: 2a279b1c71eecf13296fdf752e6edee01c986f51

See https://stackoverflow.com/questions/48137927/git-head-lowercase-vs-head-uppercase