cat-file: use strbuf_add_uint()

cat-file: use strbuf_add_uint()

Speed up printing of objectsize atoms by using the specialized function
strbuf_add_uint() instead of the general-purpose function strbuf_addf():

Benchmark 1: ./git_main cat-file --batch-all-objects --batch-check='%(objectsize)'
  Time (mean ± σ):     751.7 ms ±   1.5 ms    [User: 733.5 ms, System: 17.1 ms]
  Range (min … max):   750.5 ms … 755.0 ms    10 runs

Benchmark 2: ./git cat-file --batch-all-objects --batch-check='%(objectsize)'
  Time (mean ± σ):     720.4 ms ±   0.4 ms    [User: 701.9 ms, System: 16.7 ms]
  Range (min … max):   719.7 ms … 721.2 ms    10 runs

Summary
  ./git cat-file --batch-all-objects --batch-check='%(objectsize)' ran
    1.04 ± 0.00 times faster than ./git_main cat-file --batch-all-objects --batch-check='%(objectsize)'

Benchmark 1: ./git_main cat-file --batch-all-objects --batch-check='%(objectsize:disk)'
  Time (mean ± σ):     404.6 ms ±   0.9 ms    [User: 397.8 ms, System: 5.7 ms]
  Range (min … max):   403.3 ms … 405.9 ms    10 runs

Benchmark 2: ./git cat-file --batch-all-objects --batch-check='%(objectsize:disk)'
  Time (mean ± σ):     378.3 ms ±   0.9 ms    [User: 371.2 ms, System: 5.9 ms]
  Range (min … max):   376.8 ms … 380.2 ms    10 runs

Summary
  ./git cat-file --batch-all-objects --batch-check='%(objectsize:disk)' ran
    1.07 ± 0.00 times faster than ./git_main cat-file --batch-all-objects --batch-check='%(objectsize:disk)'

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>