ls-files: use strbuf_add_uint()

ls-files: use strbuf_add_uint()

Speed up printing of objectsize values by using the specialized function
strbuf_add_uint() as well as strbuf_insert() for padding instead of the
general-purpose function strbuf_addf().  Here are the numbers I get when
listing files in the Linux kernel repo:

Benchmark 1: ./git_main -C ../linux ls-files --format='%(objectsize)'
  Time (mean ± σ):     257.3 ms ±   0.4 ms    [User: 197.4 ms, System: 56.7 ms]
  Range (min … max):   256.7 ms … 258.1 ms    11 runs

Benchmark 2: ./git -C ../linux ls-files --format='%(objectsize)'
  Time (mean ± σ):     253.4 ms ±   0.3 ms    [User: 193.6 ms, System: 56.6 ms]
  Range (min … max):   253.0 ms … 253.8 ms    11 runs

Benchmark 3: ./git_main -C ../linux ls-files --format='%(objectsize:padded)'
  Time (mean ± σ):     257.9 ms ±   0.3 ms    [User: 198.0 ms, System: 56.6 ms]
  Range (min … max):   257.3 ms … 258.5 ms    11 runs

Benchmark 4: ./git -C ../linux ls-files --format='%(objectsize:padded)'
  Time (mean ± σ):     254.6 ms ±   1.0 ms    [User: 194.6 ms, System: 56.7 ms]
  Range (min … max):   253.7 ms … 256.8 ms    11 runs

Summary
  ./git -C ../linux ls-files --format='%(objectsize)' ran
    1.00 ± 0.00 times faster than ./git -C ../linux ls-files --format='%(objectsize:padded)'
    1.02 ± 0.00 times faster than ./git_main -C ../linux ls-files --format='%(objectsize)'
    1.02 ± 0.00 times faster than ./git_main -C ../linux ls-files --format='%(objectsize:padded)'

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