apply: convert root string to strbuf

apply: convert root string to strbuf

We use manual computation and strcpy to allocate the "root"
variable. This would be much simpler using xstrfmt.  But
since we store the length, too, we can just use a strbuf,
which handles that for us.

Note that we stop distinguishing between "no root" and
"empty root" in some cases, but that's OK; the results are
the same (e.g., inserting an empty string is a noop).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>