build: Map % back to :

Although we do not have any epochs (and should never need one!), it
serves as an example script for others to use.

get-version

@@ -22,9 +22,9 @@ if [ -f .dist-version ]; then
   # Get the version from the file distributed in the tarball.
   version=$(cat .dist-version)
 elif [ -d .git ]; then
-  # Get the version from the git repository. Since tags can't contain
-  # tildes, we use underscore instead. Reverse that switch here.
-  version=$(git describe --abbrev=4 HEAD 2>/dev/null | tr _ ~)
+  # Get the version from the git repository. Since tags cannot contain colons
+  # or tildes, we use percent and underscore instead. Reverse that switch here.
+  version=$(git describe --abbrev=4 HEAD 2>/dev/null | tr %_ :~)
 
   # Check if we are on a dirty checkout.
   git update-index --refresh -q >/dev/null