Allow to interdiff with a pre-made ref

src/main.rs

@@ -238,6 +238,13 @@ struct FormatPatch {
         help = "Reference for the interdiff (defaults to ${config.interdiff_base})"
     )]
     base_diff: Option<String>,
+    #[arg(
+        short = 'D',
+        long,
+        help = "Perform the interdiff with an explicit reference",
+        conflicts_with = "diff"
+    )]
+    diff_to: Option<String>,
     extra_args: Vec<String>,
 }
 
@@ -345,7 +352,10 @@ impl FormatPatch {
             })
         };
 
-        let _version_dir = if let Some(interdiff) = self.diff {
+        let _version_dir = match (self.diff, self.diff_to) {
+            (None, None) => format_patch(&[])?,
+            (Some(_), Some(_)) => unreachable!(),
+            (Some(patch_version), None) => {
                 let base = self
                     .base_diff
                     .or(config.interdiff_base)
@@ -411,7 +421,7 @@ impl FormatPatch {
                 wt.exec(&["switch", branch.name])?;
 
                 let patches = branch_dir
-                .join(&interdiff.to_string())
+                    .join(&patch_version.to_string())
                     .read_dir()
                     .into_diagnostic()
                     .wrap_err("Could not read interdiff folder")?
@@ -439,8 +449,10 @@ impl FormatPatch {
 
                 let interdiff_branch = format!("--interdiff={}", branch.name);
                 format_patch(&[&interdiff_branch])?
-        } else {
-            format_patch(&[])?
+            }
+            (None, Some(diff_to)) => {
+                format_patch(&[&format!("--interdiff={diff_to}")])?
+            },
         };
 
         let cover_letter = branch_dir.join(COVER_LETTER_NAME);