Have xtragc x-tra feature work in bare repos as well

dotfiles/gittum/bin/xtragc

 # Perform extra aggressive garbage collection
 #
 
+XTRAGC_REFDIR=.
+test -d ".git" && XTRAGC_REFDIR=".git"
 
-du -hs .git
+du -hs "$XTRAGC_REFDIR"
 git \
     -c gc.reflogExpire=0 \
     -c gc.reflogExpireUnreachable=0 \
     -c gc.rerereunresolved=0 \
     -c gc.pruneExpire=now \
     gc
-du -hs .git
+du -hs "$XTRAGC_REFDIR"