Add support for --force in pull

The documentation described how --force should work in pull, but
it was not implemented. This caused subrepo to display error
messages that did not work when changing branches that deviated
and an ancestory could not be cleaning tracked.

Fix #336

Author: admorgan

lib/git-subrepo

@@ -524,6 +524,12 @@ subrepo:init() {
 subrepo:pull() {
   CALL subrepo:fetch
 
+  # If forced pull, then clone instead
+  if $force_wanted; then
+    CALL subrepo:clone
+    return
+  fi
+
   # Check if we already are up to date
   # If the -u flag is present, always perform the operation
   if [[ $upstream_head_commit == $subrepo_commit ]] && ! $update_wanted; then