feat: Concat does not generate mutant when both operands are the same

[michalbundyra]

Closes #1601

This PR:

• Adds new feature - see Empty diff - Concat swap #1601
• Covered by tests
• Doc PR: not needed

[maks-rafalko]

Looks good.

IMO psalm's issues can be manually ignored in this case.

Also, probably it will be safer to do

- if ($printer->prettyPrint([$node]) !== $printer->prettyPrint([$newNode])) {
+ if ($printer->prettyPrint([clone $node]) !== $printer->prettyPrint([$newNode])) {

since $node potentially can be modified?

[maks-rafalko]

could you please ignore these Psalm errors? and we are good to merge

[michalbundyra]

@maks-rafalko done

[michalbundyra]

Not sure how it should be done then...

with

/** @psalm-suppress ImpureMethodCall */

issues were suppressed, but CS failed...

with:

/* @psalm-suppress ImpureMethodCall */

cs passes, but psalm issues are not suppressed....

[maks-rafalko]

then let's do it in psalm.xml:

<?xml version="1.0"?>
<psalm
    errorLevel="8"
    resolveFromConfigFile="true"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="https://getpsalm.org/schema/config"
    xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
    errorBaseline="psalm-baseline.xml">
    <projectFiles>
        <directory name="src/Mutator" />
        <ignoreFiles>
            <directory name="vendor" />
           <directory name="src/PhpParser" />
        </ignoreFiles>
    </projectFiles>

+    <issueHandlers>
+        <ImpureMethodCall>
+            <errorLevel type="suppress">
+                <file name="src/Mutator/Operator/Concat.php"></file>
+            </errorLevel>
+        </ImpureMethodCall>
+    </issueHandlers>
</psalm>

[maks-rafalko]

Thank you @michalbundyra

cool first contribution