version.txt: test if local changes lead to file content changes

[alxndrsn]

#1660 might be a good starting point.

Expected behaviour

given an original version.txt
when local root, client or server dirs have changes not recorded by git
and version.txt is regenerated
then new version.txt content should be different from original version.txt content

[matthew-white]

I just noticed while reviewing #1665 that some calls to git describe specify --dirty, while others do not. So maybe that's the answer? Looks like we specify --dirty for Sentry, but not for version.txt. That sounds well enough to me for now: -dirty is useful information in Sentry, whereas maybe it's not doing much in version.txt. We can always revisit that decision in the future if we need to, but maybe it's fine to run with it for now.