reftable: convert from strbuf to reftable_buf

Convert the reftable library to use the `reftable_buf` interface instead
of the `strbuf` interface. This is mostly a mechanical change via sed(1)
with some manual fixes where functions for `strbuf` and `reftable_buf`
differ. The converted code does not yet handle allocation failures. This
will be handled in subsequent commits.

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

reftable/system.h

@@ -13,7 +13,6 @@ https://developers.google.com/open-source/licenses/bsd
 
 #include "git-compat-util.h"
 #include "lockfile.h"
-#include "strbuf.h"
 #include "tempfile.h"
 #include "hash.h" /* hash ID, sizes.*/
 #include "dir.h" /* remove_dir_recursively, for tests.*/