utils/inreplace: allow non-global substitution

[cho-m]

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Can replace some occurrences of:

inreplace <file> do |s|
  s.sub! before, after
end

Also increase test coverage.

[Bo98]

Do you have examples of formulae that intentionally do this and produce wrong results under gsub? Makes the regex sound somewhat fragile.

[MikeMcQuaid]

This makes sense to me as an API. If we're using sub! already in 3+ formulae and this can simplify things there: we should do it.

[cho-m]

Do you have examples of formulae that intentionally do this and produce wrong results under gsub? Makes the regex sound somewhat fragile.

There should be ways to change regex to work under gsub, but if we know that there is and only ever will be one substitution, then it makes sense to use sub.

Though we won't get full advantage of single sub unless we provide an option to disable multiline matches and do line-by-line scan of file. However, modified files usually aren't large enough that this matters.