adding git rev as a header keyword fails, even when it's actually in a git repo

[weatherhead99]

The logic that checks whether the ".git" folder exists, only checks in the immediate current folder. Thus, for e.g. our wdlfiles repo where each project is in a subfolder, it thinks it is not inside a git repo even when it is.

technically not a WDL issue since this logic is baked into the makefiles of individual projects. At minimum, then, fix to the sample makefile is needed.
Ideally, since this is a very useful feature I would like to see WDL actually doing this itself using something like "pygit2" library to detect this rather than doing it manually in makefiles.