Fix link_to_gfm with only a reference having the incorrect link
Before:
link_to_gfm("#4", "/foo") # => "<a href="/namespace/project/issues/4" class="gfm gfm-issue">#4</a>"
link_to_gfm("See #4", "/foo") # => "<a href="/foo">See </a><a href="/namespace/project/issues/4" class="gfm gfm-issue">#4</a>"
After:
link_to_gfm("#4", "/foo") # => "<a href="/foo">#4</a>"
link_to_gfm("See #4", "/foo") # => "<a href="/foo">See </a><a href="/namespace/project/issues/4" class="gfm gfm-issue">#4</a>"
Closes #192 (closed)
Closes #1721 (closed)
Closes GitHub #7115