ugrep v7.0.2

Fix #434 ugrep v7 regression bug: an issue with the regex ^ anchor due to a misaligned begin-of-line position in the buffer after the buffer shifts on large input files. As a result, the -v option did not always work properly on large input files when searching with regex ^ anchors. Reverted to the old logic. Included a minor change to handle hex context lines #428. All extensively tested.