Merge branch 'tb/precompose-prefix-simplify'

Streamline the codepath to fix the UTF-8 encoding issues in the
argv[] and the prefix on macOS.

* tb/precompose-prefix-simplify:
  macOS: precompose startup_info->prefix
  precompose_utf8: make precompose_string_if_needed() public

diff --cc git-compat-util.h
Simple merge

diff --cc git.c
Simple merge