replace(::String, count=1) with empty string regression

[mortenpi]

On 1.4.2:

julia> replace("xyx", "" => '_', count=1)
"_xyx"

vs. 1.5.0 and master:

julia> replace("xyx", "" => '_', count=1)
"_x_y_x_"

[mortenpi]

Git bisect blames a6e27b3 from #32348. I assume though that it's just revealing a bug in replace.

cc @sostock

[sostock]

The problem is this line:

julia/base/strings/util.jl

Line 524 in 7c0cb30

r == 0:-1 || n == count && break

r == 0:-1 was never true with the old behavior, but now 0:-1 is equal to any empty range. There was a similar bug with split (which was caught by the tests and thus fixed in #32348). I can prepare a fix.

[vtjnash]

This is a side-effect of the larger bug #36768