Use previous count when 'x' command is repeated

Use previous count when 'x' command is repeated

About the 'x' command, the manual says:

    If you use <RET> to repeat the 'x' command, the repeat count N is
    used again; the other arguments default as for successive uses of
    'x'.

However, PR gdb/22619 points out that this does not work.

This patch fixes the problem.

ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	PR gdb/22619:
	* printcmd.c (last_count): New global.
	(x_command): Use saved count when repeating.

testsuite/ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	PR gdb/22619:
	* gdb.base/long_long.exp (gdb_test_long_long): Add test for repeat
	behavior.