format-patch: allow a non-integral version numbers

format-patch: allow a non-integral version numbers

The `-v<n>` option of `format-patch` can give nothing but an
integral iteration number to patches in a series.  Some people,
however, prefer to mark a new iteration with only a small fixup
with a non integral iteration number (e.g. an "oops, that was
wrong" fix-up patch for v4 iteration may be labeled as "v4.1").

Allow `format-patch` to take such a non-integral iteration
number.

`<n>` can be any string, such as '3.1' or '4rev2'. In the case
where it is a non-integral value, the "Range-diff" and "Interdiff"
headers will not include the previous version.

Signed-off-by: ZheNing Hu <adlternative@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>