worktree: inline `worktree_ref()` into its only caller

worktree: inline `worktree_ref()` into its only caller

We have `strbuf_worktree_ref()`, which works on a strbuf, and a wrapper
for it, `worktree_ref()` which returns a string. We even make this
wrapper available through worktree.h. But it only has a single caller,
sitting right next to it in worktree.c.

Just inline the wrapper into its only caller. This means the caller can
quite naturally reuse a single strbuf. We currently achieve something
similar by having a static strbuf in the wrapper.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>