patch-diff-report-tool: make ability to show lines as yellow if changes is only in message

taken from https://github.com/checkstyle/checkstyle/pull/3774

Example:
```
Summary:
Report id Files Unique rows Severity-warning
base 890 1558 1558
patch 890 35 35
difference 272 1583 1583
```

in this PR I expected no new violation, only changes.
But report forced me to review all 35 or 1558 items. What if I missed smth during scroll ?

Tool generate green and red only for initial simplicity with original author as first version.
Looks like it is time to do a bit more smart analysis of compare.
it will be very good to have "yellow" color that show what is changed. There a bunch of compare algorithms online to make this magic.

```
# warning LocalVariableName Local variable name 'dS' must match pattern '^[a-z][a-z0-9][a-zA-Z0-9]*$'. 1444 27
# warning LocalVariableName Local variable name 'dS' must match pattern '^[a-z]([a-z0-9][a-zA-Z0-9]*)?$'. 1444 27
```

should be one line in report. Diff could be shown by any means. Ideal is (italic should strikedout):

Local variable name 'dS' must match pattern '_^[a-z][a-z0-9][a-zA-Z0-9]*$_\^\[a-z\](\[a-z0-9\]\[a-zA-Z0-9\]\*\)\?\$'

but simple combination of two messages in one cell will work more than enough!

Start by locating the patch-diff-report-tool and its report comparison and rendering entry points; no specific files or tests are named in the issue. Define how message-only differences should be consolidated and displayed as yellow, then verify that unchanged violations no longer require the same review effort.

Written by the indexing model from the issue text.