Merge branch 'rs/strbuf-add-uint' into jch

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()

diff --cc builtin/cat-file.c
Simple merge

diff --cc builtin/ls-files.c
Simple merge

diff --cc strbuf.c
Simple merge