Add 'ctime' -- commit-based, and make it default

src/include/facts.sh

         latest_sha)
             git log -1 --pretty=format:%h HEAD
             ;;
+        latest_cdate)
+            git log -1 --format=%cd --date=unix HEAD
+            ;;
         *)
             warn "unknown git fact asked: $fact_name"
             ;;
     case $MKIT_TTAG in
         none)   btime= ;;
         btime)  btime=$(date -u +%Y%m%d%H%M%S) ;;
+        ctime)  btime=$(date -d @"$(git_fact latest_cdate)" -u +%Y%m%d%H%M%S) ;;
     esac
     grep ":" <<<"$prerl" \
      && warn "colon in project:prerl may corrupt version data: $prerl"

src/include/vars.sh.skel

 #
 # Add time-based ordinal tag to SemVer build data?
 #
-# Can be 'none' or 'btime'.
+# Can be 'none', 'ctime' or 'btime'.
 #
-# If 'btime', devel builds have also timestamp-based tag in format of
+# If 'ctime', devel builds have also timestamp-based tag in format of
 # `t%Y%m%d%H%M%S`, that is, a small character 't' followed by timestamp
 # without non-digit characters.  The timestamps are in UTC, ie. timezones
-# need not apply.
+# need not apply.  'btime' has the same format, except that it's derived
+# from build time, while 'ctime' is from last commit's commit date.
 #
 # This helps with deploying development builds where packaging system
 # is not SemVer-compliant and makes it hard to install arbitrary version.
 # considered same) this tag will make it more likely to "win" the build
 # you made later.
 #
-# Note that this makes devel and dirty builds non-deterministic,
-# but does not affect clean builds (ie. builds from clean repo
-# with HEAD corresponding to latest version tag.).
+# Note that this does not affect clean builds (ie. builds from clean
+# repo with HEAD corresponding to latest version tag.).
 #
-MKIT_TTAG=${MKIT_TTAG:-btime}
+# Also note that 'btime' makes the version non-deterministic: merely
+# initiating the build a second later will result in different version.
+#
+MKIT_TTAG=${MKIT_TTAG:-ctime}
 
 #
 # This MKit version