ls-tree: use strbuf_add_uint()

ls-tree: 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 objects in the Linux kernel repo:

Benchmark 1: ./git_main -C ../linux ls-tree -r --format='%(objectsize)' HEAD
  Time (mean ± σ):     294.4 ms ±   0.4 ms    [User: 231.5 ms, System: 59.4 ms]
  Range (min … max):   293.9 ms … 295.0 ms    10 runs

Benchmark 2: ./git -C ../linux ls-tree -r --format='%(objectsize)' HEAD
  Time (mean ± σ):     291.2 ms ±   0.4 ms    [User: 227.9 ms, System: 62.1 ms]
  Range (min … max):   290.6 ms … 292.0 ms    10 runs

Benchmark 3: ./git_main -C ../linux ls-tree -r --format='%(objectsize:padded)' HEAD
  Time (mean ± σ):     295.3 ms ±   0.6 ms    [User: 232.0 ms, System: 59.6 ms]
  Range (min … max):   294.3 ms … 296.3 ms    10 runs

Benchmark 4: ./git -C ../linux ls-tree -r --format='%(objectsize:padded)' HEAD
  Time (mean ± σ):     291.9 ms ±   0.4 ms    [User: 228.5 ms, System: 61.5 ms]
  Range (min … max):   291.2 ms … 292.3 ms    10 runs

Summary
  ./git -C ../linux ls-tree -r --format='%(objectsize)' HEAD ran
    1.00 ± 0.00 times faster than ./git -C ../linux ls-tree -r --format='%(objectsize:padded)' HEAD
    1.01 ± 0.00 times faster than ./git_main -C ../linux ls-tree -r --format='%(objectsize)' HEAD
    1.01 ± 0.00 times faster than ./git_main -C ../linux ls-tree -r --format='%(objectsize:padded)' HEAD

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