patch 8.1.0562: parsing of 'diffopt' is slightly wrong

Problem:    Parsing of 'diffopt' is slightly wrong.
Solution:   Fix the parsing and add a test. (Jason Franklin, Christian
            Brabandt)
diff --git a/src/testdir/dumps/Test_diff_11.dump b/src/testdir/dumps/Test_diff_11.dump
index 0e4268e..3b24ebb 100644
--- a/src/testdir/dumps/Test_diff_11.dump
+++ b/src/testdir/dumps/Test_diff_11.dump
@@ -17,4 +17,4 @@
 | +0#0000e05#a8a8a8255@1|~+0#4040ff13#ffffff0| @33||+1#0000000&| +0#0000e05#a8a8a8255@1|~+0#4040ff13#ffffff0| @33
 | +0#0000e05#a8a8a8255@1|~+0#4040ff13#ffffff0| @33||+1#0000000&| +0#0000e05#a8a8a8255@1|~+0#4040ff13#ffffff0| @33
 |X+3#0000000&|f|i|l|e|1| @12|1|,|0|-|1| @9|A|l@1| |X+1&&|f|i|l|e|2| @12|1|,|0|-|1| @9|A|l@1
-|:+0&&|s|e|t| |d|i|f@1|o|p|t|+|=|i|n|d|e|n|t|-|h|e|u|r|i|s|t|i|c| @44
+|:+0&&| @73