builtin/reflog: respect user config in "write" subcommand

builtin/reflog: respect user config in "write" subcommand

The reflog write recognizes only GIT_COMMITTER_NAME and
GIT_COMMITTER_EMAIL environment variables, but forgot to honor the
user.name and user.email configuration variables, due to lack of
repo_config() call to grab these values from the configuration files.

The test suite sets these variables, so this behavior was unnoticed.

Ensure that the reflog write also uses the values of user.name and
user.email if set in the Git configuration.

Co-authored-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Michael Lohmann <git@lohmann.sh>
Signed-off-by: Junio C Hamano <gitster@pobox.com>