showlinenum : Update output format documentation

- Move the output format documentation to its own section, explain the
format and add an example.
showlinenum : Accept files missing newline at EOF

- Accept the backslash indicator in diff lines, which as far as I know
is only used to warn when a file is missing a newline at EOF.

--- a/nonewline.txt
+++ b/nonewline.txt
@@ -1 +1 @@
 :-a
 :\ No newline at end of file
1:+b
 :\ No newline at end of file

Prior to this change the backslash indicator would cause a fatal error.
showlinenum : Fix check for combined diff abort

- Fix the combined diff detection from exactly 3 @ chars to at least 3.

According to git diff's manual, "There are (number of parents + 1) @
characters in the chunk header for combined diff format."

Prior to this change the combined diff format wasn't identified if there
were more than two parents.