Suggestion: Add `diff` method

Create a patch file for the suggestion. Instead of building the patch
text manually, as in my earlier attempts (`patch` and `unified_diff`),
apply the patch to the file in memory, and then ask libgit2 to give us a
Git patch by comparing the original blob and the patched buffer.

Doing this because I was unsuccessful in manually composing a patch file
that Git would accept. I could create unified diffs that the `patch`
program would accept, but these wouldn't work with `git-apply`. My
current guess is that these didn't work because I didn't have both
before and after context in my manually-created patch files.

Since this tool is intended to be used with Git, I want a patch file
that will work transparently with Git utilities. The easiest alternative
to manually generating the patch file seemed to be to have Git create
the patch text.

Add a new binary, `git-sugpatch`, that outputs the patch text to
standard output.

TODO: This gets the original from the current HEAD in `apply_to()`. We
should instead be using the contents of `blob`, because the original is
not necessarily the working copy.

2 files changed, 63 insertions, 2 deletions

diff --git a/src/bin/git-sugpatch.rs b/src/bin/git-sugpatch.rs
new file mode 100644
index 0000000..08d1a38
--- /dev/null
+++ b/src/bin/git-sugpatch.rs
@@ -0,0 +1,25 @@
+use std::env;
+use std::process;
+
+use git_suggested_patch::{Client, SuggestionUrl};
+
+
+fn main() {
+    let args: Vec<_> = env::args().collect();
+
+    if args.len() < 2 {
+        process::exit(111);
+    }
+
+    let url: SuggestionUrl = args[1].parse().unwrap();
+
+    let client = Client::new(
+        env!("GITHUB_TOKEN"),
+        &url.owner,
+        &url.repo,
+    );
+
+    let suggestion = client.fetch(&url.comment_id).unwrap();
+
+    print!("{}", suggestion.diff());
+}
diff --git a/src/lib.rs b/src/lib.rs
index f894413..0be907a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -8,10 +8,10 @@ pub use crate::url::SuggestionUrl;
 
 use std::fs;
 use std::fs::{File, OpenOptions};
-use std::io::{BufRead, BufReader, Write};
+use std::io::{BufRead, BufReader, BufWriter, Write};
 use std::path::Path;
 
-use git2::Repository;
+use git2::{Patch, Repository};
 use github_rs::client::{Executor, Github};
 use regex::Regex;
 use serde::Deserialize;
@@ -80,6 +80,9 @@ pub struct Suggestion {
     #[serde(rename = "body")]
     comment: String,
 
+    #[serde(rename = "original_commit_id")]
+    commit: String,
+
     path: String,
 
     original_start_line: Option<usize>,
@@ -89,6 +92,7 @@ pub struct Suggestion {
 }
 
 impl Suggestion {
+    // TODO: Rename to `diff`
     pub fn patch(&self) -> String {
         let mut diff: Vec<_> = self.diff.lines()
             .filter(|l| !l.starts_with("-"))
@@ -110,6 +114,38 @@ impl Suggestion {
         diff.join("\n")
     }
 
+    pub fn diff(&self) -> String {
+        let repo = Repository::open(".").unwrap();
+        let commit = repo.find_commit(self.commit.parse().unwrap()).unwrap();
+
+        let path = Path::new(&self.path);
+
+        let object = commit
+            .tree().unwrap()
+            .get_path(path).unwrap()
+            .to_object(&repo).unwrap();
+
+        let blob = object.as_blob().unwrap();
+
+        let mut new = BufWriter::new(Vec::new());
+        self.apply_to(&self.path, &mut new).unwrap();
+        let new_buffer = new.into_inner().unwrap();
+
+        let mut diff = Patch::from_blob_and_buffer(
+            blob,
+            Some(&path),
+            &new_buffer,
+            Some(&path),
+            None,
+        ).unwrap();
+
+        diff.to_buf()
+            .unwrap()
+            .as_str()
+            .unwrap_or("")
+            .to_owned()
+    }
+
     fn suggestion_patch(&self) -> String {
         let re = Regex::new(r"(?s).*(?-s)```\s*suggestion.*\n").unwrap();
         let s = re.replace(&self.comment, "+");