For visual selections spanning multiple lines, keep caret position
if it's on the first line. Otherwise move the caret to the start of
the first selected line.
Note that this implementation assumes that the 'gU' / 'gu' command in
visual mode is equivalent to 'U' / 'u'. While 'v_gU' and 'v_gu' are not
explicitly documented in Vim help, we treat these commands as identical
based on observed behavior, without examining Vim's source code.
Xinhe Wang