`sync-template` trashes GitHub expressions

[sellout]

GitHub expressions look like ${{ <expression> }}. sync-template uses Mustache to populate the template, which replaces {{…}}. This results in any GitHub expressions in the template being turned into $, which doesn’t work.

Some options

• escape non-Mustache {{ somehow so they end up as {{ in the finished template;
• have Mustache ignore {{…}} that doesn’t contain a known value (rather than using the empty string); or
• use a different templating system than Mustache.