grep: extract grep_binexp() from grep_or_expr()

grep: extract grep_binexp() from grep_or_expr()

When constructing an OR node, the grep.c code uses `grep_or_expr()` to
make a node, assign its kind, and set its left and right children. The
same is not done for AND nodes.

Prepare to introduce a new `grep_and_expr()` function which will share
code with the existing implementation of `grep_or_expr()` by introducing
a new function which compiles either kind of binary expression, and
reimplement `grep_or_expr()` in terms of it.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>