Allow for merging data files when reloading

This allows for merging the (unsaved) items with the items from the data
files when invoking the reload operation. To this end, an external merge
tool (defaults to vimdiff) is used.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/vars.c

@@ -126,7 +126,7 @@ struct dmon_conf dmon;
  */
 void vars_init(void)
 {
-	const char *ed, *pg;
+	const char *ed, *pg, *mt;
 
 	/* Variables for user configuration */
 	conf.confirm_quit = 1;
@@ -159,6 +159,11 @@ void vars_init(void)
 		pg = DEFAULT_PAGER;
 	conf.pager = pg;
 
+	mt = getenv("MERGETOOL");
+	if (mt == NULL || mt[0] == '\0')
+		mt = DEFAULT_MERGETOOL;
+	conf.mergetool = mt;
+
 	wins_set_layout(1);
 
 	ui_calendar_set_first_day_of_week(MONDAY);