readline: avoid memcpy() of overlapping regions

memcpy() for overlapping regions is undefined behavior; use memmove()
instead in readline_hist_add().

[Keep tab characters since surrounding code still uses them -- Stefan]

Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
