t/lib-pager.sh: remove unnecessary '^' from 'expr' regular expression

Regular expressions matched by 'expr' have an implicit '^' at the beginning of them and so are anchored to the beginning of the string. Using the '^' character to mean "match at the beginning", is redundant and could produce the wrong result if 'expr' implementations interpret the '^' as a literal '^'. Additionally, GNU expr 5.97 complains like this: expr: warning: unportable BRE: `^[a-z][a-z]*$': using `^' as the first character of the basic regular expression is not portable; it is being ignored Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil> Signed-off-by: Junio C Hamano <gitster@pobox.com>