Confirm targets exist before linking

Targets *might* exist in Git, but not in the work tree.
(especially when initially cloning)

yadm

@@ -117,11 +117,13 @@ function alt() {
   #; for every file which matches the above regex, create a symlink
   for tracked_file in $(git ls-files | sort); do
     tracked_file="$YADM_WORK/$tracked_file"
-    if [[ $tracked_file =~ $match ]] ; then
-      new_link="${BASH_REMATCH[1]}"
-      debug "Linking $tracked_file to $new_link"
-      [ -n "$LOUD" ] && echo "Linking $tracked_file to $new_link"
-      ln -fs "$tracked_file" "$new_link"
+    if [ -e "$tracked_file" ] ; then
+      if [[ $tracked_file =~ $match ]] ; then
+        new_link="${BASH_REMATCH[1]}"
+        debug "Linking $tracked_file to $new_link"
+        [ -n "$LOUD" ] && echo "Linking $tracked_file to $new_link"
+        ln -fs "$tracked_file" "$new_link"
+      fi
     fi
   done