RFR: JDK-8294564: IGV: IllegalArgumentException for "Difference to current graph" [v3]

[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. 
> 
> ---------
> ### Progress
> - [ ] Change must be properly reviewed (1 review required, with at least 1 [Reviewer](https://openjdk.org/bylaws#reviewer))
> - [x] Change must not contain extraneous whitespace
> - [x] Commit message must refer to an issue
> 
> 
> 
> ### Reviewing
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> `$ git checkout pull/10533`
> 
> Update a local copy of the PR: \
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Tobias Holenstein has updated the pull request incrementally with two additional commits since the last revision:

 - split lines
 - remove empty line

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/10533/files
  - new: https://git.openjdk.org/jdk/pull/10533/files/f5aacd3b..ff0477b4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10533&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10533&range=01-02

  Stats: 5 lines in 2 files changed: 2 ins; 1 del; 2 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