Add devshell and initial bats tests

test/mechanicaldiff.bats

@@ -0,0 +1,50 @@
+#!/usr/bin/env bats
+
+setup() {
+  set -euo pipefail
+
+  bats_load_library bats-support
+  bats_load_library bats-assert
+
+  DIR="$(cd "$(dirname "$BATS_TEST_FILENAME")" >/dev/null 2>&1 && pwd)"
+  PATH="$DIR/../bin:$PATH"
+}
+
+@test "keeps hunk when replacement matches" {
+  run bash -c 'cat <<"EOF" | mechanicaldiff.py foo bar
+diff --git a/file.txt b/file.txt
+index 1111111..2222222 100644
+--- a/file.txt
++++ b/file.txt
+@@ -1,1 +1,1 @@
+-foo
++bar
+EOF'
+  assert_success
+  expected="$(
+    cat <<'EOF'
+diff --git a/file.txt b/file.txt
+index 1111111..2222222 100644
+--- a/file.txt
++++ b/file.txt
+@@ -1,1 +1,1 @@
+-foo
++bar
+EOF
+  )"
+  assert_output "$expected"
+}
+
+@test "drops diff when replacement does not match" {
+  run bash -c 'cat <<"EOF" | mechanicaldiff.py foo bar
+diff --git a/file.txt b/file.txt
+index 1111111..2222222 100644
+--- a/file.txt
++++ b/file.txt
+@@ -1,1 +1,1 @@
+-foo
++baz
+EOF'
+  assert_success
+  assert_output ""
+}