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>

config.c

@@ -1676,7 +1676,9 @@ static int do_git_config_sequence(const struct config_options *opts,
 		repo_config = NULL;
 
 	current_parsing_scope = CONFIG_SCOPE_SYSTEM;
-	if (git_config_system() && !access_or_die(git_etc_gitconfig(), R_OK, 0))
+	if (git_config_system() && !access_or_die(git_etc_gitconfig(), R_OK,
+						  opts->system_gently ?
+						  ACCESS_EACCES_OK : 0))
 		ret += git_config_from_file(fn, git_etc_gitconfig(),
 					    data);
 
@@ -1807,6 +1809,7 @@ void read_very_early_config(config_fn_t cb, void *data)
 	opts.ignore_repo = 1;
 	opts.ignore_worktree = 1;
 	opts.ignore_cmdline = 1;
+	opts.system_gently = 1;
 
 	config_with_options(cb, data, NULL, &opts);
 }

config.h

@@ -58,6 +58,7 @@ struct config_options {
 	unsigned int ignore_repo : 1;
 	unsigned int ignore_worktree : 1;
 	unsigned int ignore_cmdline : 1;
+	unsigned int system_gently : 1;
 	const char *commondir;
 	const char *git_dir;
 	config_parser_event_fn_t event_fn;