Fix PR cli/18053

PR cli/18053 concerns a couple of minor bugs in the JIT debuginfo
support.  First, jit-reader-load should use filename completion and
support tilde expansion.  Second, the help for jit-reader-unload is
incorrect.  While working on this I also realized that
jit-reader-unload should use the no-op completer, so I've included
that as well.

Built and regtested on x86-64 Fedora 23.  A completer test for
jit-reader-load is included, but not a tilde-expansion test, as I
couldn't think of a reliable way to test that.

2016-07-13  Tom Tromey  <tom@tromey.com>

	PR cli/18053:
	* jit.c (jit_reader_load_command): Use tilde_expand.
	(_initialize_jit): Fix help for jit-reader-unload.  Set completer
	for new commands.

2016-07-13  Tom Tromey  <tom@tromey.com>

	PR cli/18053:
	* gdb.base/jit-so.exp (one_jit_test): Add jit-reader-load
	completion test.