Allow unpadded days in patch headers (#14)
A closer look at the Git source shows that the rfc2822 and default
formats do not zero-pad days, so we need to accept single-digit days
when parsing. The single-digit pattern will also accept the padded
format, so there's no loss of functionality.
Change the parsing test to use single-digit values to emphasize when
padding is required and when it is not.
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