config: respect commondir

Worktrees present an interesting problem when it comes to the config.
Historically we could assume that the per-repository config lives at
'gitdir/config', but since worktrees were introduced this isn't the case
anymore.  There is currently no way to specify per-worktree
configuration, and as such the repository config is shared with all
worktrees and is located at 'commondir/config'.

Many users of the config machinery correctly set
'config_options.git_dir' with the repository's commondir, allowing the
config to be properly loaded when operating in a worktree.  But other's,
like 'read_early_config()', set 'config_options.git_dir' with the
repository's gitdir which can be incorrect when using worktrees.

To fix this issue, and to make things less ambiguous, lets add a
'commondir' field to the 'config_options' struct and have all callers
properly set both the 'git_dir' and 'commondir' fields so that the
config machinery is able to properly find the repository's config.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: bmwill

config.c

@@ -1531,8 +1531,8 @@ static int do_git_config_sequence(const struct config_options *opts,
 	char *user_config = expand_user_path("~/.gitconfig", 0);
 	char *repo_config;
 
-	if (opts->git_dir)
-		repo_config = mkpathdup("%s/config", opts->git_dir);
+	if (opts->commondir)
+		repo_config = mkpathdup("%s/config", opts->commondir);
 	else if (have_git_dir())
 		repo_config = git_pathdup("config");
 	else
@@ -1644,7 +1644,8 @@ void read_early_config(config_fn_t cb, void *data)
 
 	opts.respect_includes = 1;
 
-	if (have_git_dir())
+	if (have_git_dir()) {
+		opts.commondir = get_git_common_dir();
 		opts.git_dir = get_git_dir();
 	/*
 	 * When setup_git_directory() was not yet asked to discover the
@@ -1654,8 +1655,10 @@ void read_early_config(config_fn_t cb, void *data)
 	 * notably, the current working directory is still the same after the
 	 * call).
 	 */
-	else if (!discover_git_directory(&commondir, &gitdir))
+	} else if (!discover_git_directory(&commondir, &gitdir)) {
+		opts.commondir = commondir.buf;
 		opts.git_dir = gitdir.buf;
+	}
 
 	git_config_with_options(cb, data, NULL, &opts);
 

config.h

@@ -30,6 +30,7 @@ enum config_origin_type {
 
 struct config_options {
 	unsigned int respect_includes : 1;
+	const char *commondir;
 	const char *git_dir;
 };