Notmuch, like org, uses git describe to build a version string variable
at install time, but Doom preforms shallow clones of its packages, where
git describe does not work.
Notmuch is one of our bigger packages and the variable isn't essential
for normal use, so I've opted to build notmuch-version.el by hand. It
lacks the commit offset, but it has enough information to work with.
Fix#5581
473eda795a