Fix ref-search button display

[Hirse]

Fix what @jung-kim intended to fix in #1311, but got lost in the whitespace-refactor:
Only show search button if there are hidden refs.

It was included in the first commit of that PR, but subsequently lost:

ungit/components/graph/graph.html

Line 44 in 80fb354

<!-- ko if: refs().length > ungit.config.numRefsToShow -->

---

Also fixes an issue with the selection of tags and branches to display:
For numRefsToShow = 5, numBranchesToShow = 3, and numTagsToShow = 2,
only enforce the branch and tag numbers if the total number of refs exceeds the limit.
Examples:

• 3 branches and 2 tags -> 3 branches and 2 tags
• 5 branches and 1 tag -> 4 branches and 1 tag
• 5 branches and 2 tags -> 3 branches and 2 tags
• 2 branches and 5 tags -> 2 branches and 3 tags
  (Previously, the tag limit would have been enforced in this case leading to only 2 tags being displayed, and with the search button change, the 3. tag being not discoverable.)