main(): Open repository even if PWD is a subdirectory

Previously, running `git todo` outside the root of the repository would
result in this error:

    error: unable to open repository: could not find repository from '.'; class=Repository (6); code=NotFound (-3)

It turns out the `open()` method expects the given path to be the
repository root. Use `open_from_env()` instead, which functions more
like regular Git commands.

2 files changed, 69 insertions, 1 deletions

diff --git a/src/main.rs b/src/main.rs
index 1a41001..775dfae 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -50,7 +50,7 @@ fn main() {
         process::exit(exitcode::OK);
     }
 
-    let repo = match Repository::open(".") {
+    let repo = match Repository::open_from_env() {
         Ok(r) => r,
         Err(e) => {
             eprintln(&format!("unable to open repository: {}", e));
diff --git a/t/104-works-outside-the-repo-root.t b/t/104-works-outside-the-repo-root.t
new file mode 100644
index 0000000..d61b5c6
--- /dev/null
+++ b/t/104-works-outside-the-repo-root.t
@@ -0,0 +1,68 @@
+#!/usr/bin/env perl -w
+
+# Copyright (c) 2020  Teddy Wing
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+
+use strict;
+
+use File::Copy;
+use Test::More;
+
+use Bin qw($BIN);
+
+my $file = 'git-sugdiff.rs';
+
+chdir 't-git-repo' or die $!;
+
+system('git checkout -b fork-point');
+ok !$?;
+
+open(my $input, '<', $file) or die $!;
+open(my $output, '>', "$file.out") or die $!;
+
+while (<$input>) {
+	if ($. == 34) {
+		print $output "    // TODO: 100-shows-todo-comments-since-fork-point\n";
+	}
+
+	print $output $_;
+}
+
+close $input;
+close $output;
+
+move("$file.out", $file) or die $!;
+
+system('git add git-sugdiff.rs');
+ok !$?;
+
+system('git commit -m "New TODO"');
+ok !$?;
+
+mkdir 'subdir' or die $!;
+chdir 'subdir' or die $!;
+
+my $todos = qx($BIN);
+is $todos, 'git-sugdiff.rs:34:    // TODO: 100-shows-todo-comments-since-fork-point
+';
+
+
+# Teardown
+system('git checkout master');
+system('git branch -D fork-point');
+
+
+done_testing;