Panic when processing diff file with only file renames.

[psandana]

Description
Generating a diff file with git:

git diff origin/main.. | Tee-Object diff.out

Generates the following output:

diff --git a/crate/examples/example1.rs b/crate/examples/example_renamed1.rs
similarity index 100%
rename from crate/examples/example1.rs
rename to /crate/examples/example_renamed1.rs

The code is panicking on:

stack backtrace:
   0:     0x7ff699b03aa2 - std::sys_common::backtrace::_print::impl$0::fmt
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\std\src\sys_common\backtrace.rs:44
   1:     0x7ff699b279cd - core::fmt::rt::Argument::fmt
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\core\src\fmt\rt.rs:142
   2:     0x7ff699b279cd - core::fmt::write
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\core\src\fmt\mod.rs:1120
   3:     0x7ff699aff6a1 - std::io::Write::write_fmt<std::sys::windows::stdio::Stderr>
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\std\src\io\mod.rs:1810
   4:     0x7ff699b038ca - std::sys_common::backtrace::_print
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\std\src\sys_common\backtrace.rs:47
   5:     0x7ff699b038ca - std::sys_common::backtrace::print
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\std\src\sys_common\backtrace.rs:34
   6:     0x7ff699b06029 - std::panicking::default_hook::closure$1
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\std\src\panicking.rs:272
   7:     0x7ff699b05ce5 - std::panicking::default_hook
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\std\src\panicking.rs:292
   8:     0x7ff699b06554 - std::panicking::rust_panic_with_hook
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\std\src\panicking.rs:779
   9:     0x7ff699b06429 - std::panicking::begin_panic_handler::closure$0
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\std\src\panicking.rs:657
  10:     0x7ff699b043a9 - std::sys_common::backtrace::__rust_end_short_backtrace<std::panicking::begin_panic_handler::closure_env$0,never$>
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\std\src\sys_common\backtrace.rs:171
  11:     0x7ff699b060f2 - std::panicking::begin_panic_handler
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\std\src\panicking.rs:645
  12:     0x7ff699b32547 - core::panicking::panic_fmt
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\core\src\panicking.rs:72
  13:     0x7ff6995678da - patch::parser::parse_multiple_patches
                               at C:\Users\user\.cargo\registry\src\index.crates.io-6f17d22bba15001f\patch-0.7.0\src\parser.rs:84
  14:     0x7ff6995672fb - patch::ast::Patch::from_multiple
                               at C:\Users\user\.cargo\registry\src\index.crates.io-6f17d22bba15001f\patch-0.7.0\src\ast.rs:134
  15:     0x7ff6991a0641 - cargo_mutants::in_diff::diff_filter
                               at C:\open-src\cargo-mutants\src\in_diff.rs:23
  16:     0x7ff699136a25 - cargo_mutants::main

I was using cargo mutants, but this is reproducible by using patch too. patch::parser::patch is not processing errors from the interior, which are valid: Not finding the --- tags because diff is only renames.

Ask
Add error checks to gracefully handle cases when there is no changes.