set errno=0 before strtoX calls

set errno=0 before strtoX calls

To detect conversion failure after calls to functions like `strtod`, one
can check `errno == ERANGE`. These functions are not guaranteed to set
`errno` to `0` on successful conversion, however. Manual manipulation of
`errno` can likely be avoided by checking that the output pointer
differs from the input pointer, but that's not how other locations, such
as parse.c:139, handle this issue; they set errno to 0 prior to
executing the function.

For every place I could find a strtoX function with an ERANGE check
following it, set `errno = 0;` prior to executing the conversion
function.

Signed-off-by: Kyle Lippincott <spectral@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>