fix a side effect of the plugin: the last substitute string is overwritten

[liushapku]

…itten

Before the change the call to s:MatchesInRange sets the last substitute string to ''

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[liushapku]

I signed it

[googlebot]

CLAs look good, thanks!

[rburny]

Thank you!

[dbarnett]

Could also benefit from a comment to clarify what it does. Anyway, +1 thanks!