Merge branch 'jk/reading-packed-refs'

Merge branch 'jk/reading-packed-refs'

An earlier rewrite to use strbuf_getwholeline() instead of fgets(3)
to read packed-refs file revealed that the former is unacceptably
inefficient.

* jk/reading-packed-refs:
  t1430: add another refs-escape test
  read_packed_refs: avoid double-checking sane refs
  strbuf_getwholeline: use getdelim if it is available
  strbuf_getwholeline: avoid calling strbuf_grow
  strbuf_addch: avoid calling strbuf_grow
  config: use getc_unlocked when reading from file
  strbuf_getwholeline: use getc_unlocked
  git-compat-util: add fallbacks for unlocked stdio
  strbuf_getwholeline: use getc macro