Respect the XDG standard for configuration files

From GH #513:

    Rather than reading the user configuration from `~/.tigrc` by
    default, instead check `$XDG_CONFIG_HOME/tig/config` and only then
    look for `~/.tigrc`. Further reading:
    https://ploum.net/207-modify-your-application-to-use-xdg-folders/

Fixes #513

NEWS.adoc

@@ -34,6 +34,7 @@ Improvements:
  - Compact relative date display mode. (GH #331)
  - Add date column option controlling whether to show local date.
  - Move to parent commit in the main view. (GH #388)
+ - Respect the XDG standard for configuration files. (GH #513)
  - Support for custom `strftime(3)` date formats, e.g.:
 
 	set main-view-date = custom

src/options.c

@@ -958,6 +958,7 @@ load_options(void)
 	const char *tig_diff_opts = getenv("TIG_DIFF_OPTS");
 	const bool diff_opts_from_args = !!opt_diff_options;
 	bool custom_tigrc_system = !!tigrc_system;
+	char buf[SIZEOF_STR];
 
 	opt_file_filter = true;
 	if (!find_option_info_by_value(&opt_diff_context)->seen)
@@ -977,8 +978,17 @@ load_options(void)
 			return error("Error in built-in config");
 	}
 
-	if (!tigrc_user)
-		tigrc_user = TIG_USER_CONFIG;
+	if (!tigrc_user) {
+		const char *xdg_config_home = getenv("XDG_CONFIG_HOME");
+
+		if (!xdg_config_home)
+			tigrc_user = TIG_USER_CONFIG;
+		else if (!string_format(buf, "%s/tig/config", xdg_config_home))
+			return error("Failed to expand $XDG_CONFIG_HOME");
+		else
+			tigrc_user = buf;
+	}
+
 	load_option_file(tigrc_user);
 
 	if (!diff_opts_from_args && tig_diff_opts && *tig_diff_opts) {

test/tigrc/xdg-config-home-test

@@ -0,0 +1,28 @@
+#!/bin/sh
+
+. 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