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

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>