worktree: refactor infer_backlink() to use *strbuf

This lays the groundwork for the next patch, which needs the backlink
returned from infer_backlink() as a `strbuf`. It seemed inefficient to
convert from `strbuf` to `char*` and back to `strbuf` again.

This refactors infer_backlink() to return an integer result and use a
pre-allocated `strbuf` for the inferred backlink path, replacing the
previous `char*` return type and improving efficiency.

Signed-off-by: Caleb White <cdwhite3@pm.me>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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