Added even more quick exit in InsertAttrRuns (#5644)

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## Summary of the Pull Request

In tonight's episode of "I wanna my CPU back", we'll see a quite familiar face whose name is Mr.AttrRow.

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## References

#2937

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## PR Checklist
* [ ] Closes #xxx
* [ ] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [ ] Requires documentation to be updated
* [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

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## Detailed Description of the Pull Request / Additional comments

I knew this is possible a long time ago. Just didn't got the chance to actually implement this. I understand that you guys are busy preparing the v1.0 release. So if this is a bad time, this can wait.

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## Validation Steps Performed

Manually tested. If all goes well, nothing will be broken.

.github/actions/spell-check/whitelist/whitelist.txt

@@ -1,5 +1,7 @@
 AAAAAABBBBBBCCC
 AAAAABBBBBBBCCC
+AAAAABCCCCCCCCC
+AAAAADCCCCCCCCC
 AAD
 ABANDONFONT
 ABCDEFGHIJKLMNO

src/buffer/out/AttrRow.cpp

@@ -280,7 +280,7 @@ void ATTR_ROW::ReplaceAttrs(const TextAttribute& toBeReplacedAttr, const TextAtt
         }
         // .. otherwise if we internally have a list of 2 or more and we're about to insert a single color
         // it's possible that we just walk left-to-right through the row and find a quick exit.
-        else if (iStart > 0 && iStart == iEnd)
+        else if (iStart >= 0 && iStart == iEnd)
         {
             // First we try to find the run where the insertion happens, using lowerBound and upperBound to track
             // where we are currently at.
@@ -306,8 +306,22 @@ void ATTR_ROW::ReplaceAttrs(const TextAttribute& toBeReplacedAttr, const TextAtt
                         return S_OK;
                     }
 
+                    // If the current run has length of exactly one, we can simply change the attribute
+                    // of the current run.
+                    // e.g.
+                    // AAAAABCCCCCCCCC
+                    //      ^
+                    // AAAAADCCCCCCCCC
+                    //
+                    // Here 'D' is the new color.
+                    if (curr->GetLength() == 1)
+                    {
+                        curr->SetAttributes(NewAttr);
+                        return S_OK;
+                    }
+
                     // If the insertion happens at current run's lower boundary...
-                    if (iStart == lowerBound)
+                    if (iStart == lowerBound && i > 0)
                     {
                         const auto prev = std::prev(curr, 1);
                         // ... and the previous run has the same color as the new one, we can
@@ -332,6 +346,32 @@ void ATTR_ROW::ReplaceAttrs(const TextAttribute& toBeReplacedAttr, const TextAtt
                             return S_OK;
                         }
                     }
+
+                    // If the insertion happens at current run's upper boundary...
+                    if (iStart == upperBound - 1 && i + 1 < _list.size())
+                    {
+                        // ...then let's try our luck with the next run if possible. This is basically the opposite
+                        // of what we did with the previous run.
+                        // e.g.
+                        // AAAAAABBBBBBCCC
+                        //      ^
+                        // AAAAABBBBBBBCCC
+                        //
+                        // Here 'B' is the new color.
+                        const auto next = std::next(curr, 1);
+                        if (NewAttr == next->GetAttributes())
+                        {
+                            curr->DecrementLength();
+                            next->IncrementLength();
+
+                            if (curr->GetLength() == 0)
+                            {
+                                _list.erase(curr);
+                            }
+
+                            return S_OK;
+                        }
+                    }
                 }
 
                 // Advance one run in the _list.