cli: use a unique file extension for description tempfiles

Establishing a unique file extension for the temporary files created
via `jj describe` helps to ensure that text editors can recognize the
filetype and alter settings accordingly. This will open the door for
an improved user experience, and allow for setting things like the
appropriate text-width/rulers, syntax highlighting of the diff summary
(see Git's commit tree-sitter grammer [1]), easy toggling of the `JJ:`
comment lines, etc.

I examined the behavior of filetype detection across a number of
common text editors, and the most universally-support mechanism was
to have a unique extension that does not include any periods. Meaning
that namespacing via something like `.jj.txt` instead, won't always be
detected due to inconsistent matching prioritization across editors.
It also makes sense to assume that we may want other Jujutsu-specific
filetypes in the future.

The filename prefix has also been switched to be `editor-` for clarity,
as well as to ease matching a glob-pattern if we ever need to garbage
collect leftover tempfiles. This structure is similar to what Mercurial
and Sapling do as well.

[1] https://github.com/the-mikedavis/tree-sitter-git-commit

CHANGELOG.md

@@ -82,6 +82,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * `jj` with no subcommand now defaults to `jj log` instead of showing help. This
   command can be overridden by setting `ui.default-command`.
 
+* Description tempfiles created via `jj describe` now have the file extension
+  `.jjdescription` to help external tooling detect a unique filetype.
+
 ### Fixed bugs
 
 * Modify/delete conflicts now include context lines

src/commands/mod.rs

@@ -1769,8 +1769,8 @@ fn edit_description(
 ) -> Result<String, CommandError> {
     let description_file_path = (|| -> Result<_, io::Error> {
         let mut file = tempfile::Builder::new()
-            .prefix("description-")
-            .suffix(".txt")
+            .prefix("editor-")
+            .suffix(".jjdescription")
             .tempfile_in(repo.repo_path())?;
         file.write_all(description.as_bytes())?;
         file.write_all(b"\nJJ: Lines starting with \"JJ: \" (like this one) will be removed.\n")?;