Support pulling out the original base of the PR body to better preserve history

src/api/pull_request.rs

@@ -1,3 +1,4 @@
+use regex::Regex;
 use serde::Deserialize;
 use serde::Serialize;
 use std::error::Error;
@@ -70,6 +71,20 @@ impl PullRequest {
         &self.base.gitref
     }
 
+    /// Stored version of the base branch in the pull request body.
+    /// Needed for when the original branch is merged and so it vanishes.
+    pub fn base_branch_orig(&self) -> Option<&str> {
+        let regex = Regex::new("git-stack-base-branch:([^ ]+)").unwrap();
+        self.body
+            .as_ref()
+            .map(|body| {
+                regex
+                    .captures(&body)
+                    .map(|re_match| re_match.get(1).unwrap().as_str())
+            })
+            .flatten()
+    }
+
     pub fn url(&self) -> &str {
         &self.url
     }

src/graph.rs

@@ -16,7 +16,8 @@ pub fn build(prs: &[Rc<PullRequest>]) -> Graph<Rc<PullRequest>, usize> {
 
     for (i, pr) in prs.iter().enumerate() {
         let head_handle = handles[i];
-        if let Some(&base_handle) = handles_by_head.get(pr.base()) {
+        let base_to_use = pr.base_branch_orig().unwrap_or(pr.base());
+        if let Some(&base_handle) = handles_by_head.get(base_to_use) {
             tree.add_edge(*base_handle, head_handle, 1);
         }
     }