Merge branch 'jk/shorten-unambiguous-ref-wo-sscanf'

sscanf(3) used in "git symbolic-ref --short" implementation found
to be not working reliably on macOS in UTF-8 locales.  Rewrite the
code to avoid sscanf() altogether to work it around.

* jk/shorten-unambiguous-ref-wo-sscanf:
  shorten_unambiguous_ref(): avoid sscanf()
  shorten_unambiguous_ref(): use NUM_REV_PARSE_RULES constant
  shorten_unambiguous_ref(): avoid integer truncation

diff --cc refs.c
Simple merge