reftable: stop using `strbuf_addf()`

reftable: stop using `strbuf_addf()`

We're about to introduce our own `reftable_buf` type to replace
`strbuf`. One function we'll have to convert is `strbuf_addf()`, which
is used in a handful of places. This function uses `snprintf()`
internally, which makes porting it a bit more involved:

  - It is not available on all platforms.

  - Some platforms like Windows have broken implementations.

So by using `snprintf()` we'd also push the burden on downstream users
of the reftable library to make available a properly working version of
it.

Most callsites of `strbuf_addf()` are trivial to convert to not using
it. We do end up using `snprintf()` in our unit tests, but that isn't
much of a problem for downstream users of the reftable library.

While at it, remove a useless call to `strbuf_reset()` in
`t_reftable_stack_auto_compaction_with_locked_tables()`. We don't write
to the buffer before this and initialize it with `STRBUF_INIT`, so there
is no need to reset anything.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Taylor Blau <me@ttaylorr.com>