[patchbomb] prepend leading zeros in the "[PATCH N of M]" string
Without this patch, if one tries to send a patch bomb with more than 9
patches, the resulting subjects are not easily alphabetically sortable.
For example:
...
[PATCH 9 of 10]
[PATCH 10 of 10]
This patch prepends as many leading zeros as necessary. E.g.,
[PATCH 09 of 10]
or
[PATCH 009 of 100]