version.sh: append -dirty if the working tree contains modifications

This allows us to detect users who run mpv with custom patches.

version.sh

@@ -34,7 +34,7 @@ fi
 # or from "git describe" output
 git_revision=$(cat snapshot_version 2> /dev/null)
 test "$git_revision" || test ! -e .git || git_revision="$(git describe \
-    --match "v[0-9]*" --always --tags | sed 's/^v//')"
+    --match "v[0-9]*" --always --tags --dirty | sed 's/^v//')"
 version="$git_revision"
 
 # other tarballs extract the version number from the VERSION file