diff: more carefully search for corresponding VirtualFile
diff: use more complex highlighter in fragmented diff
diff: better painting for fragmented diff

add \n at the end of fake document, if it is not the end of original document

Thus we expand background highlighting till the end of line
diff: better highlighting in diff - merge panel
diff: better highlighting in 3-way diff

Show basic highlighting (FileType-based)
diff: fix gutter width calculation: line number max width

* case of empty line number (because of lineNumberConvertor) of the last line in document
EA-58610