report: Merge entries have same name

Multiple entries can have same name because they might be from different
library (--nest-libcall), same binary but different session or C++ name
demangling.  It'd be better merging them and show as a single item.

Closes #154.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>

cmd-report.c

@@ -60,6 +60,8 @@ static void insert_entry(struct rb_root *root, struct trace_entry *te, bool thre
 
 		if (thread)
 			cmp = te->pid - entry->pid;
+		else if (te->sym && entry->sym)
+			cmp = strcmp(te->sym->name, entry->sym->name);
 		else
 			cmp = te->addr - entry->addr;