trace2: fixup access problem on /etc/gitconfig in read_very_early_config

trace2: fixup access problem on /etc/gitconfig in read_very_early_config

Teach do_git_config_sequence() to optionally gently check for access to
the system config.  Use this option in read_very_early_config() when
initializing trace2.

In [1] SZEDER Gábor reported that my changes in [2] introduced a
regression when the user does not have permission to read the system
config.

This commit addresses that problem by optionally ignoring that error.

[1] https://public-inbox.org/git/285beb2b2d740ce20fdd8af1becf371ab39703db.1554995916.git.gitgitgadget@gmail.com/T/#m342e839289aec515523a98b5e34d7f42d3f1fd79
[2] https://public-inbox.org/git/285beb2b2d740ce20fdd8af1becf371ab39703db.1554995916.git.gitgitgadget@gmail.com/T/#m11b59c9228c698442f750ee8f9b10c629399ae48

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>