fix clang -Wtautological-compare with unsigned enum

fix clang -Wtautological-compare with unsigned enum

Create a GREP_HEADER_FIELD_MIN so we can check that the field value is
sane and silence the clang warning.

Clang warning happens because the enum is unsigned (this is
implementation-defined, and there is no negative fields) and the check
is then tautological.

Signed-off-by: Antoine Pelisse <redacted>
Signed-off-by: John Keeping <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>