grep: inline the return value of a function call used only once

grep: inline the return value of a function call used only once

Since e944d9d932 ("grep: rewrite an if/else condition to avoid
duplicate expression", 2016-06-25) the "ascii_only" variable has only
been used once in compile_regexp(), let's just inline it there.

This makes the code easier to read, and might make it marginally
faster depending on compiler optimizations.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>