RFR: JDK-8294564: IGV: IllegalArgumentException for "Difference to current graph"

[Tobias Holenstein]

"Difference to current graph" opens a new window with a difference graph in IGV. Unfortunately, it was throwing a  `java.lang.IllegalArgumentException`

# Overview 
In IGV, for every opened graph, that is not a difference graph, the user can right-click on any other graph in the Outline and select "Difference to current graph". This opens a difference graph showing the difference between the currently opened graph and the selected graph. 
<img width="478" alt="difference to current" src="https://user-images.githubusercontent.com/71546117/193585135-8e93cb2f-6c05-40c4-9b19-7253ce8d6bd8.png">

If the current graph is already a difference graph, the function is disabled. Same if the selected graph is identical to the opened one. 
<img width="508" alt="difference disabled" src="https://user-images.githubusercontent.com/71546117/193585932-4444bb46-7354-4138-81b6-1b11cfad4a8b.png">

# Implementation
The problem was that the difference graph did not keep track of which two `InputGraphs` it was based on. Therefore the functions `getFirstGraph()` and `getSecondGraph` in `DiagramViewModel` did not work properly. Now, `InputGraph` keeps track of the first and second `InputGraph` if it is a difference graph (`isDiffGraph`). And `getFirstGraph()` and `getSecondGraph` are updated to return the right `InputGraph`s for difference graphs. 

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### Progress
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- [x] Change must not contain extraneous whitespace
- [x] Commit message must refer to an issue



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Commit messages:
 - no difference graph for same graph
 - inline hasEditor()
 - bug fix isOpen()
 - Update OutlineTopComponent.java
 - safe firstGraph and secondGraph in Inputgraph for diff graphs
 - new GraphViewerImplementation  view_difference()
 - findEditorForGraph
 - no diffgraph of a diffgraph
 - IGV: IllegalArgumentException for "Difference to current graph"

Changes: https://git.openjdk.org/jdk/pull/10533/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10533&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294564
  Stats: 174 lines in 9 files changed: 94 ins; 21 del; 59 mod
  Patch: https://git.openjdk.org/jdk/pull/10533.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10533/head:pull/10533

PR: https://git.openjdk.org/jdk/pull/10533