worktree: do not pass strbuf by value

worktree: do not pass strbuf by value

write_worktree_linking_files() takes two struct strbuf parameters by
value, even though it only reads path strings from them.

Passing a strbuf by value is misleading and dangerous. The structure
carries a pointer to its underlying character array; caller and callee
end up sharing that storage.  If the callee ever causes the strbuf to
be reallocated, the caller's copy becomes a dangling pointer, which
results in a double-free when the caller does strbuf_release().

The function only needs the string values, not the strbuf machinery.
Switch it to take const char * and update all callers to pass .buf.

Signed-off-by: Deveshi Dwivedi <deveshigurgaon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>