When generating git-diff options, lei-rediff extracts the single
character option from the lei option spec. However, there's no check
that the regular expression actually matches, leading to an
unintentional git-diff option when there isn't a short option (e.g.,
--inter-hunk-context=1 maps to the invalid `git diff --color -w1').
Check for a match before trying to extract the single character
option.
Fixes: cf0c7ce3ce81b5c3 (lei rediff: regenerate diffs from stdin)