replace strbuf_addstr(git_path()) with git_path_buf()

replace strbuf_addstr(git_path()) with git_path_buf()

Writing directly into the strbuf avoids a useless copy of
the data, and dropping calls to git_path() makes it easier
to audit for dangerous calls.

Note that git_path() does an implicit strbuf_reset(), but in
each of these cases we were either already doing that reset,
or writing into a fresh strbuf anyway.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>