Set the default target branch based on init.defaultBranch

Fixes #6.

src/config.rs

@@ -0,0 +1,26 @@
+use std::path::Path;
+
+use anyhow::Result;
+use git_commands::git;
+
+use crate::trim::TrimAsciiWhitespace;
+
+/// Get the default branch name from Git config's `init.defaultBranch` setting,
+/// falling back to 'master' if it isn't set. This should help handle default
+/// branch names that are more awoke.
+pub fn default_branch_name(for_path: &Path) -> Result<String> {
+    let output = git(
+        &[
+            "--no-pager",
+            "config",
+            "--default",
+            "master",
+            "--get",
+            "init.defaultBranch",
+        ],
+        for_path,
+    )?
+    .stdout;
+    let output = std::str::from_utf8(output.trim_ascii_whitespace())?;
+    Ok(output.to_owned())
+}

src/lib.rs

@@ -9,6 +9,8 @@ use std::{
     time::{SystemTime, UNIX_EPOCH},
 };
 
+mod config;
+use config::*;
 mod conflicts;
 use conflicts::*;
 mod glob;
@@ -42,7 +44,7 @@ fn set_committer_date_to_now() {
 ///
 pub fn autorebase(
     path: &Path,
-    onto_branch: &str,
+    onto_branch: Option<&str>,
     slow_conflict_detection: bool,
     include_non_local: bool,
     match_branches: Option<&str>,
@@ -52,6 +54,16 @@ pub fn autorebase(
         bail!("Your Git installation is too old - version 2.23 or later is required");
     }
 
+    // Get the target branch name in this priority order:
+    //
+    // 1. Set explicitly via `--onto`
+    // 2. The `init.defaultBranch` git config setting.
+    // 3. "master"
+    let onto_branch = match onto_branch {
+        Some(b) => b.to_owned(),
+        None => default_branch_name(path)?,
+    };
+
     // The first thing we do is set the commiter date to now. If we don't do this
     // then when we have two branch labels on the same commit, when they get
     // rebased they will be given different commiter dates which will mean they
@@ -91,7 +103,8 @@ pub fn autorebase(
     let onto_branch_info = all_branches
         .iter()
         .find(|b| b.branch == onto_branch)
-        .ok_or_else(|| anyhow!("Couldn't find target branch '{}'", onto_branch))?;
+        .ok_or_else(|| anyhow!("Couldn't find target branch '{}'. You can set the default target \
+                                    branch via 'git config init.defaultBranch' or use the --onto flag.", onto_branch))?;
     eprintln!("\r{}", "• Getting branches...".green());
 
     // Print a summary of the branches, and simultaneously filter them.
@@ -140,7 +153,7 @@ pub fn autorebase(
             &git_common_dir,
             &mut conflicts,
             &conflicts_path,
-            onto_branch,
+            &onto_branch,
             &autorebase_worktree_path,
             slow_conflict_detection,
         )?;

src/main.rs

@@ -11,9 +11,10 @@ use std::env::current_dir;
 /// Automatically pull the master branch and rebase all branches without
 /// upstreams onto it.
 struct CliOptions {
-    /// the target branch to pull and rebase onto (typically "master" or "develop")
-    #[argh(option, default = "String::from(\"master\")")]
-    onto: String,
+    /// the target branch to pull and rebase onto;
+    /// defaults to `git config --get init.defaultBranch` or `master` if unset
+    #[argh(option)]
+    onto: Option<String>,
 
     /// if there are conflicts, try rebasing commit by commit backwards from the
     /// target, instead of trying to determind the conflicting commit on the
@@ -53,7 +54,7 @@ fn run() -> Result<()> {
 
     autorebase(
         &current_dir()?,
-        &options.onto,
+        options.onto.as_deref(),
         options.slow,
         options.include_non_local,
         options.match_branches.as_deref(),