Merge branch 'bw/config-h'

This backports f31d23a (Merge branch 'bw/config-h', 2017-06-24) from
upstream Git's `master` branch, plus a patch that seems to be required
to let the test suite pass.

This topic branch fixes problems when looking up aliases in worktrees.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

config.c

@@ -1542,6 +1542,8 @@ static int do_git_config_sequence(const struct config_options *opts,
 
 	if (opts->commondir)
 		repo_config = mkpathdup("%s/config", opts->commondir);
+	else if (opts->git_dir)
+		repo_config = mkpathdup("%s/config", opts->git_dir);
 	else
 		repo_config = NULL;