revision: forbid combining --graph and --no-walk

revision: forbid combining --graph and --no-walk

Because "--graph" is about connected history while --no-walk is
about discrete points, it does not make sense to allow these two
options at the same time. [1]

This change makes a few calls to "show --graph" fail in t4052, but
asking to show one commit with graph is a nonsensical thing to do.
Thus, tests on "show --graph" in t4052 have been removed [2,3].
Same tests on "show" without --graph option have already been tested
in 4052.

3 testcases have been added to test this patch.

[1]: http://article.gmane.org/gmane.comp.version-control.git/216083
[2]: http://article.gmane.org/gmane.comp.version-control.git/264950
[3]: http://article.gmane.org/gmane.comp.version-control.git/265107

Helped-By: Eric Sunshine <sunshine@sunshineco.com>
Helped-By: René Scharfe <l.s.r@web.de>
Helped-By: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Dongcan Jiang <dongcan.jiang@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>