replace history is broken for empty strings and/or duplicate entries

[Tim Van Holder]

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Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

If a replacement is done to the empty string (i.e. if replace-string and
its ilk are used to delete text), the replacement history does not cope
well.
It also ignores replacements where both sides are identical (an edge case,
admittedly, as such replacements are pointless; however, I feel even
pointless replacements should be kept in the history).

Example:

M-x replace-string RET {no history         } x RET y RET
M-x replace-string RET {history: x -> y    } y RET RET
M-x replace-string RET {history: x -> y (!)} z RET RET
M-x replace-string RET {history: y -> z (!)} z RET z RET
M-x replace-string RET {history: y -> z (!)}

Both issues seem to have their root in read-from-minibuffer, as the
history contains ("z" "y" "x") in the above example.  As such there
is probably no easy fix, other than adding two new arguments to
read-from-minibuffer (KEEP-EMPTY-STRINGS and KEEP-DUPLICATES or
similar).