#5219 [string replace] refuses to replace empty substrings

[Don Porter]

Can't believe I missed this. Should have been part
of the Do Nothing Gracefully cleanup.

You cannot use [string replace] to insert a substring
without at the same time taking at least on character
out.

% string replace abcde 1 0 h
abcde

Instead of the result any sane programmer would
expect:

ahbcde

In particular, the useful case of prepending is not
supported.

Recategorize to a Feature Request if you must. (This
misfeature is documented, FFS). But it's so boneheaded
it looks like a bug to me.

[Don Porter]

See branch bug-3608714.

Surprising thing is that the test suite does not care.