Simplify dirt detection code using git_bool()

src/include/facts.sh

     if git_present;
     then    # we are in git repo
         local last_hash=$(git rev-parse HEAD)
-        if test "$(git diff --shortstat 2>/dev/null)" != "";
-        then    # the tree is "dirty", i.e. has been edited
-            local suffix=.dirty
-        fi
-        local suffix=""
-        echo -n "$last_hash$suffix"
+        echo -n "$last_hash"
+        git_bool dirty && echo -n ".dirty"
     else    # we are outside (eg. distributor's build dir')
         grep . .mkit/git_lasthash || {
             echo UNKNOWN
         then    # we are at a later commit than the last tag
             local commit="$(git_fact current_branch).g$(git_fact latest_sha)"
         fi
-        if test "$(git diff --shortstat 2>/dev/null)" != "";
-        then    # the tree is "dirty", i.e. has been edited
-            local dirty=dirty
-        fi
-        test -n "$latest_tag" && version=${latest_tag:1}
+        local dirty=""
         local suffix=""
-        case "$commit:$dirty" in
-            :)       suffix=""                ;;
-            :dirty)  suffix="+$dirty"         ;;
-            *:)      suffix="+$commit"        ;;
-            *:dirty) suffix="+$commit.$dirty" ;;
+        git_bool dirty; dirty=$?
+        test -n "$latest_tag" && version=${latest_tag:1}
+        case "$dirty:$commit" in
+            1:)  suffix=""               ;;
+            0:)  suffix="+dirty"         ;;
+            1:*) suffix="+$commit"       ;;
+            0:*) suffix="+$commit.dirty" ;;
+            *)   suffix=MKIT_BUG;
+                 warn "MKIT_BUG: bad dirt/commit detection" ;;
         esac
         test -b "$prerl" && suffix="-$prerl$suffix"
         version="$version$suffix"