Merge branch 'rs/strbuf-add-uint'

Adding a decimal integer with strbuf_addf("%u") appears commonly; they have been optimized by using a custom formatter. * rs/strbuf-add-uint: ls-tree: use strbuf_add_uint() ls-files: use strbuf_add_uint() cat-file: use strbuf_add_uint() strbuf: add strbuf_add_uint()