patch 9.1.0986: filetype: 'jj' filetype is a bit imprecise

Problem:  filetype: 'jj' filetype is a bit imprecise
Solution: rename 'jj' filetype to 'jjdescription'
          (Gregory Anders)

closes: #16364

Signed-off-by: Gregory Anders <greg@gpanders.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/syntax/jjdescription.vim b/runtime/syntax/jjdescription.vim
new file mode 100644
index 0000000..04848bc
--- /dev/null
+++ b/runtime/syntax/jjdescription.vim
@@ -0,0 +1,24 @@
+" Vim syntax file
+" Language:	jj description
+" Maintainer:	Gregory Anders <greg@gpanders.com>
+" Last Change:	2024 May 8
+
+if exists('b:current_syntax')
+  finish
+endif
+
+syn match jjAdded "A .*" contained
+syn match jjRemoved "D .*" contained
+syn match jjChanged "M .*" contained
+
+syn region jjComment start="^JJ: " end="$" contains=jjAdded,jjRemoved,jjChanged
+
+syn include @jjCommitDiff syntax/diff.vim
+syn region jjCommitDiff start=/\%(^diff --\%(git\|cc\|combined\) \)\@=/ end=/^\%(diff --\|$\|@@\@!\|[^[:alnum:]\ +-]\S\@!\)\@=/ fold contains=@jjCommitDiff
+
+hi def link jjComment Comment
+hi def link jjAdded Added
+hi def link jjRemoved Removed
+hi def link jjChanged Changed
+
+let b:current_syntax = 'jjdescription'