Ignore leading and trailing blank lines for comparison

```git-check-assertions
run test/mechanicaldiff.bats
assert_success

git checkout HEAD~ bin
run test/mechanicaldiff.bats
assert_failure
assert_output --partial "-- command failed --"
```

bin/mechanicaldiff.py

@@ -4,6 +4,16 @@ import re
 import sys
 
 
+def trim_blank_ends(lines: list[str]) -> list[str]:
+    start = 0
+    end = len(lines)
+    while start < end and lines[start] == "\n":
+        start += 1
+    while end > start and lines[end - 1] == "\n":
+        end -= 1
+    return lines[start:end]
+
+
 def filter_change_block(
     change_lines: list[str], search: str, replace: str
 ) -> list[str]:
@@ -27,7 +37,7 @@ def filter_change_block(
     transformed_removed = [
         re.sub(search, replace, line) for line in removed_lines
     ]
-    if transformed_removed == added_lines:
+    if trim_blank_ends(transformed_removed) == trim_blank_ends(added_lines):
         return change_lines
 
     if len(removed_lines) != len(added_lines):

test/mechanicaldiff.bats

@@ -143,6 +143,24 @@ setup() {
   assert_output "$(cat "$BATS_TEST_TMPDIR/diff")"
 }
 
+@test "ignores leading and trailing blank lines for comparison" {
+  cat >"$BATS_TEST_TMPDIR/old" <<-'EOF'
+
+	prefix foo suffix
+
+	EOF
+  cat >"$BATS_TEST_TMPDIR/new" <<-'EOF'
+	prefix bar suffix
+	EOF
+
+  git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
+    >"$BATS_TEST_TMPDIR/diff" || true
+
+  run mechanicaldiff.py foo bar <"$BATS_TEST_TMPDIR/diff"
+  assert_success
+  assert_output "$(cat "$BATS_TEST_TMPDIR/diff")"
+}
+
 @test "keeps only matching line in a consecutive change block" {
   cat >"$BATS_TEST_TMPDIR/old" <<-'EOF'
 	foo