patch 8.2.0019: cannot number of lines of another buffer

Problem:    Cannot number of lines of another buffer.
Solution:   Add "linecount" to getbufinfo(). (Yasuhiro Matsumoto,
            closes #5370)

runtime/doc/eval.txt

@@ -4796,6 +4796,8 @@ getbufinfo([{dict}])
 					{only with the |+viminfo| feature}
 			listed		TRUE if the buffer is listed.
 			lnum		current line number in buffer.
+			linecount	number of lines in the buffer (only
+					valid when loaded)
 			loaded		TRUE if the buffer is loaded.
 			name		full path to the file in the buffer.
 			signs		list of signs placed in the buffer.