mbox: get version indicator from "PATCHv##"
mbox: get version indicator from "PATCHv##"

If "[PATCHv2]" is used without a space before the "v", Patchew misses the
version number.  For example http://next.patchew.org/QEMU/20180326153441.32641-1-f4bug@amsat.org/
mistakes the previous "v5" as "v1".

Signed-off-by: Paolo Bonzini <69d3ebcf5ee4e7f05a50dc23cd96655b730804b4@redhat.com>

patchew-project/patchew,patchew-project/patchew,patchew-project/patchew,patchew-project/patchew