Add git command to allowlist

In Release 0.129.0 [0], hugo changed their internal implementation to
respect the allowList config when calling git. Adding this fixes the
currently failing build.

The git information is used to add the "last modified" link at the bottom of each RFC
page.

[0]: https://github.com/gohugoio/hugo/releases/tag/v0.129.0

config.toml

@@ -15,4 +15,4 @@ pluralizeListTitles = false
 
 [security]
   [security.exec]
-    allow = ['^rst2html$']
+    allow = ['^rst2html$', '^git$']