Improve XDG_CONFIG_HOME tests and document the new behaviour

doc/tig.1.adoc

@@ -222,8 +222,15 @@ TIG_NO_DISPLAY::
 
 FILES
 -----
+'$XDG_CONFIG_HOME/tig/config::
+'~/.config/tig/config'::
 '~/.tigrc'::
-	User configuration file. See manpage:tigrc[5] for examples.
+	The Tig user configuration file is loaded in the following way. If
+	`$XDG_CONFIG_HOME` is set, read user configuration from
+	`$XDG_CONFIG_HOME/tig/config`. If `$XDG_CONFIG_HOME` is empty or
+	undefined, read user configuration from `~/.config/tig/config` if it
+	exists and fall back to `~/.tigrc` if it does not exist. See
+	manpage:tigrc[5] for examples.
 
 '{sysconfdir}/tigrc'::
 	System wide configuration file.

test/tigrc/xdg-config-home-test

@@ -2,27 +2,30 @@
 
 . libtest.sh
 
-tigrc <<EOF
-This tigrc file should not be loaded!
-EOF
-
-steps "
-	<Ctrl-t>
-"
-
-#export TIGRC_SYSTEM="should-not-be-loaded"
-export XDG_CONFIG_HOME="$HOME/.config"
-mkdir -p "$XDG_CONFIG_HOME/tig"
-
-cat > "$XDG_CONFIG_HOME/tig/config" <<EOF
-bind generic <Ctrl-t> @sh -c 'echo ran > ~/RUNME'
-EOF
-
-test_tig status
-
-assert_equals stderr <<EOF
-EOF
-
-assert_equals RUNME <<EOF
-ran
-EOF
+mkdir -p "$HOME/etc/tig" "$HOME/.config/tig"
+
+check()
+{
+	name="$(echo "$1" | tr / -)"
+	path="$HOME/$1"
+	env="${2:-undefined}"
+
+	case "$env" in
+	  undefined) unset XDG_CONFIG_HOME ;;
+	  *) export "$env" ;;
+	esac
+
+	mkdir -p "$(dirname "$path")"
+	tig_script "case-$name" '<Ctrl-t>'
+	echo "bind generic <Ctrl-t> @sh -c 'echo $name >> $HOME/$TEST_NAME.out'" > "$path"
+	test_tig status
+	assert_equals "$TEST_NAME.out" "$name"
+	assert_equals "$TEST_NAME.stderr" ''
+}
+
+unset XDG_CONFIG_HOME
+
+check .tigrc
+check .config/tig/config
+check .config/tig/config XDG_CONFIG_HOME="$HOME"
+check etc/tig/config XDG_CONFIG_HOME="$HOME/etc"