hex: add and use strbuf_add_oid_hex()

hex: add and use strbuf_add_oid_hex()

Add a function for adding the full hexadecimal hash value of an object
ID to a strbuf.  It's thread-safe and slightly more efficient than using
strbuf_addstr() with oid_to_hex() because it doesn't have to determine
the length of the string or copy it from the intermediate static buffer.

Add and apply a semantic patch to use it throughout the code base.

I get a tiny speedup for git log showing a single hash per commit:

Benchmark 1: ./git_main log --format=%H
  Time (mean ± σ):      91.2 ms ±   0.7 ms    [User: 51.9 ms, System: 38.6 ms]
  Range (min … max):    89.8 ms …  92.6 ms    31 runs

Benchmark 2: ./git log --format=%H
  Time (mean ± σ):      90.5 ms ±   0.7 ms    [User: 51.0 ms, System: 38.8 ms]
  Range (min … max):    89.2 ms …  92.3 ms    32 runs

Summary
  ./git log --format=%H ran
    1.01 ± 0.01 times faster than ./git_main log --format=%H

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