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.

1 files changed, 25 insertions, 0 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());
+}