Implements a new reftable-specific strbuf replacement to reduce
reftable's dependency on Git-specific data structures.
* ps/reftable-strbuf:
reftable: handle trivial `reftable_buf` errors
reftable/stack: adapt `stack_filename()` to handle allocation failures
reftable/record: adapt `reftable_record_key()` to handle allocation failures
reftable/stack: adapt `format_name()` to handle allocation failures
t/unit-tests: check for `reftable_buf` allocation errors
reftable/blocksource: adapt interface name
reftable: convert from `strbuf` to `reftable_buf`
reftable/basics: provide new `reftable_buf` interface
reftable: stop using `strbuf_addf()`
reftable: stop using `strbuf_addbuf()`
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