calc threads cycle detection should not break computing a value

If ScFormulaCell::Interpret() is called to actually compute the cell's
value (bRunning==true), do not bail out on a detected cycle, it
needs to be detected as an actual cyclic reference. Happens with
testCopyPasteMixedReferenceFormula when during computing dependencies
cell C1 actually has a cyclic dependency on itself. Without this
change, it would simply use its previously computed value.

Change-Id: Ibf90b118b87d3536f5014004ea4a1dbad93ab270
Reviewed-on: https://gerrit.libreoffice.org/63183
Tested-by: Jenkins
Reviewed-by: Dennis Francis <dennis.francis@collabora.com>
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

sc/source/core/data/formulacell.cxx

@@ -1523,8 +1523,10 @@ void ScFormulaCell::Interpret()
 
     ScFormulaCell* pTopCell = mxGroup ? mxGroup->mpTopCell : this;
 
-    if (pTopCell->mbSeenInPath && rRecursionHelper.GetDepComputeLevel())
+    if (pTopCell->mbSeenInPath && rRecursionHelper.GetDepComputeLevel() && !bRunning)
     {
+        // This call arose from a dependency calculation and we just found a cycle.
+        // This will mark all elements in the cycle as parts-of-cycle.
         ScFormulaGroupCycleCheckGuard aCycleCheckGuard(rRecursionHelper, pTopCell);
         return;
     }