expand $left/$right in edit-args, replace all $variable matches

CHANGELOG.md

@@ -44,6 +44,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * The `diff.format` config option is now called `ui.diff.format`. The old name
   is still supported for now.
 
+* `merge-tools.<name>.edit-args` now requires `$left`/`$right` parameters.
+  The default is `edit-args = ["$left", "$right"]`.
+
 ### New features
 
 * The default log format now uses the committer timestamp instead of the author

docs/config.md

@@ -199,21 +199,31 @@ Obviously, you would only set one line, don't copy them all in!
 ## Editing diffs
 
 The `ui.diff-editor` setting affects the tool used for editing diffs (e.g.
-`jj split`, `jj amend -i`). The default is `meld`. The left and right
-directories to diff are passed as the first and second argument respectively.
+`jj split`, `jj amend -i`). The default is `meld`.
+
+`jj` makes the following substitutions:
+
+- `$left` and `$right` are replaced with the paths to the left and right
+  directories to diff respectively.
+
+If no arguments are specified, `["$left", "$right"]` are set by default.
 
 For example:
 
 ```toml
-ui.diff-editor = "kdiff3" # Use merge-tools.kdiff3.edit-args
-ui.diff-editor = ["kdiff3", "--merge"] # Specify edit-args inline
+# Use merge-tools.kdiff3.edit-args
+ui.diff-editor = "kdiff3"
+# Specify edit-args inline
+ui.diff-editor = ["kdiff3", "--merge", "$left", "$right"]
 ```
 
-Custom arguments can be added, and will be inserted before the paths to diff:
+If `ui.diff-editor` consists of a single word, e.g. `"kdiff3"`, the arguments
+will be read from the following config keys.
 
 ```toml
 # merge-tools.kdiff3.program = "kdiff3"      # Defaults to the name of the tool if not specified
-merge-tools.kdiff3.edit-args = ["--merge", "--cs", "CreateBakFiles=0"]
+merge-tools.kdiff3.edit-args = [
+    "--merge", "--cs", "CreateBakFiles=0", "$left", "$right"]
 ```
 
 ### Using Vim as a diff editor
@@ -260,7 +270,7 @@ merge-tools.vimdiff.program = "vim"
 merge-tools.vimdiff.merge-tool-edits-conflict-markers = true    # See below for an explanation
 ```
 
-`jj` replaces the following arguments with the appropriate file names:
+`jj` makes the following substitutions:
 
 - `$output` (REQUIRED) is replaced with the name of the file that the merge tool
   should output. `jj` will read this file after the merge tool exits.

src/config-schema.json

@@ -228,6 +228,12 @@
                     "program": {
                         "type": "string"
                     },
+                    "edit-args": {
+                        "type": "array",
+                        "items": {
+                            "type": "string"
+                        }
+                    },
                     "merge-args": {
                         "type": "array",
                         "items": {

src/config/merge_tools.toml

@@ -1,9 +1,10 @@
 [merge-tools.kdiff3]
 # --merge to open output pane, CreateBakFiles=0 to not include backup files in commit
-edit-args = ["--merge", "--cs", "CreateBakFiles=0"]
+edit-args = ["--merge", "--cs", "CreateBakFiles=0", "$left", "$right"]
 merge-args = ["$base", "$left", "$right", "-o", "$output", "--auto"]
 
 [merge-tools.meld]
+edit-args = ["$left", "$right"]
 merge-args = ["$left", "$base", "$right", "-o", "$output", "--auto-merge"]
 
 [merge-tools.vimdiff]
@@ -16,4 +17,4 @@ merge-tool-edits-conflict-markers = true
 # Using vimdiff as a diff editor is not recommended. For instructions on configuring
 # the DirDiff Vim plugin for a better experience, see
 # https://gist.github.com/ilyagr/5d6339fb7dac5e7ab06fe1561ec62d45
-edit-args = ["-f", "-d"]
+edit-args = ["-f", "-d", "$left", "$right"]