Diff tool: remove Save button, change "Diff" to "Compare files", try to prevent people creating invalid patches with it.

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@46027 4005530d-fff6-0310-9dd1-cebe43e6787f
Diff tool: rename variables.

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@46028 4005530d-fff6-0310-9dd1-cebe43e6787f
Diff tool: refactoring, move duplicate code to class TSelectedDiffFile.

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@46029 4005530d-fff6-0310-9dd1-cebe43e6787f