> I tried to evaluate "git rev-parse HEAD" in a build script and hand over the
> result to the build process by setting a custom variable to the hash string
> via --variable hash=${git rev-parse HEAD}. In my template I have $hash$

The syntax for including command output in a shell command line is
$(cmd), not ${cmd}. ${cmd} expands to the value of the shell variable
cmd instead of executing cmd. Your command line should thus look like

$ pandoc ... --variable "hash=$(git rev-parse HEAD)"


> I stored the hash in a file and tried to include it with -B, though this
> would not be a good solution if it worked at all.

Do you have $include-before$ somewhere in your template, if not then
pandoc doesn't know where to include the contents of the file.