Transfer common methods and nested classes to a separate utility class

Inspired from #254

We have some methods that potentially can or already are used repeatedly in multiple classes.

Some of common methods that are repeatedly used both in DiffTool.java and GitCommands.java from #254 :

https://github.com/checkstyle/test-configs/blob/22718ccf5b2fe08b6ab62ad0b0e790fc1f5e20b0/diff-java-tool/src/main/java/com/github/checkstyle/difftool/DiffTool.java#L1918-L1973

https://github.com/checkstyle/test-configs/blob/22718ccf5b2fe08b6ab62ad0b0e790fc1f5e20b0/diff-java-tool/src/main/java/com/github/checkstyle/difftool/DiffTool.java#L1139-L1182

https://github.com/checkstyle/test-configs/blob/22718ccf5b2fe08b6ab62ad0b0e790fc1f5e20b0/diff-java-tool/src/main/java/com/github/checkstyle/difftool/DiffTool.java#L2521-L2537

Read the cited ranges in DiffTool.java and compare them with the corresponding common methods and nested classes in GitCommands.java. Identify the shared responsibilities and usages before choosing the separate utility class; the refactor is done when the shared members are centralized and both callers use them without duplicated definitions.

Written by the indexing model from the issue text.