Fix latest_tag git fact parsing

although I haven't been able to put my finger on it, git-log must have
changed between Debian's 2.1.4 and Fedora's 2.5.something.  The previous
version worked on Fedora but not on Debian; hopefully this solution is
more robust.

src/include/facts.sh

     git_present || warn "can't give fact outside git repo: $fact_name"
     case "$fact_name" in
         latest_tag)
-            git log --format="%D" \
+            git log --format="%d" \
               | sed 's/,/\n/g' \
-              | sed 's/^[[:blank:]]*//; ' \
-              | grep -E '^tag: v[[:digit:]]+\.' \
-              | cut -d' ' -f2 \
-              | head -1
+              | sed 's/^[[:blank:]]*//' \
+              | grep -E '^\(?tag' \
+              | tr -cd '[[:digit:]].v\n' \
+              | grep . -m 1
             ;;
         latest_version)
             git_fact latest_tag | git_tag2ver