Better handling of "Binary files differ" and git index lines

[sourcefrog]

With this changed we can correctly parse and, to some extent roundtrip, diffs like this:

diff --git a/bin b/bin
index 4805bf6..e807873 100644
Binary files a/bin and b/bin differ

[Copilot]

Pull request overview

This PR enhances the diff parser to correctly handle binary file diffs that include git's "diff --git" and "index" lines. The implementation adds support for parsing these metadata lines before the "Binary files differ" message, enabling proper roundtrip parsing of git-generated binary diffs.

Changes:

• Added parsing for optional diff --git command lines and git index lines before binary file indicators
• Introduced a new binary field to the Patch struct to distinguish binary patches from regular ones
• Updated the Display implementation to output binary patch format appropriately

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/wild-samples/gitsmallbinary.patch	Test case for minimal binary diff with git metadata
tests/wild-samples/gitbinary.patch	Test case for binary diff with longer file paths
tests/regressions.rs	Updated test to include new binary: false field
tests/parse_samples.rs	Improved error messages and comments
src/parser.rs	Added diff_command and git_index_line parsers; enhanced binary diff parsing; improved error messages
src/ast.rs	Added binary field to Patch struct and updated Display implementation
CHANGELOG.md	Documented the new binary field

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