Add support for "bup index --exclude-rx <pattern> ...".

Add support for "bup index --exclude-rx <pattern> ...".

When --exclude-rx <pattern> is provided to bup index, exclude any path
matching <pattern>, which must be a Python regular expression
(http://docs.python.org/library/re.html).  The pattern will be
compared against the full path, without anchoring, so "x/y" will match
"ox/yard" or "box/yards".  To exclude the contents of /tmp, but not
the directory itself, use "^/tmp/.".

You may check the behavior at runtime by setting BUP_DEBUG=2 in the
environment.

Thanks to Zoran Zaric <zz@zoranzaric.de> for reporting a bug in an
earlier version of this patch.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Reviewed-by: Zoran Zaric <zz@zoranzaric.de>