macOS: precompose startup_info->prefix

macOS: precompose startup_info->prefix

The "prefix" was precomposed for macOS in commit 5c327502 (MacOS:
precompose_argv_prefix(), 2021-02-03).

However, this commit forgot to update "startup_info->prefix" after
precomposing.

Move the (possible) precomposition towards the end of
setup_git_directory_gently(), so that precompose_string_if_needed()
can use git_config_get_bool("core.precomposeunicode") correctly.

Keep prefix, startup_info->prefix and GIT_PREFIX_ENVIRONMENT all in sync.

And as a result, the prefix no longer needs to be precomposed in git.c

Reported-by: Dmitry Torilov <d.torilov@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>