Use T/F for dirt detection

When decoupled from context of exit status, 0 and 1 can be confusing.

src/include/facts.sh

     local prerl         # pre-release keyword (from mkit.ini, eg. 'beta')
     local latest_tag    # latest git tag
     local commit        # commit indicator (CURRENT_BRANCH.gHASH)
-    local dirty         # 0 if dirty, 1 if clean
+    local dirty=F       # F if dirty, T if clean
     local suffix        # version suffix
     prerl=$(ini 1value project:prerl)
     grep ":" <<<"$prerl" \
     then    # we are at a later commit than the last tag
         commit="$(git_fact current_branch).g$(git_fact latest_sha)"
     fi
-    git_bool dirty; dirty=$?
+    git_bool dirty && dirty=T
     case "$dirty:$commit" in
-        1:)  suffix=""               ;;
-        0:)  suffix="+dirty"         ;;
-        1:*) suffix="+$commit"       ;;
-        0:*) suffix="+$commit.dirty" ;;
+        F:)  suffix=""               ;;
+        T:)  suffix="+dirty"         ;;
+        F:*) suffix="+$commit"       ;;
+        T:*) suffix="+$commit.dirty" ;;
         *)   suffix=MKIT_BUG
              warn "MKIT_BUG: bad dirt/commit detection" ;;
     esac