patch: accept empty-file diffs with no hunk - #7331

What

Parsing a diff that adds an empty file fails with unexpected header line. Staging a new zero-byte file and taking a plain git diff produces a header with no hunk:

diff --git a/new_file b/new_file
new file mode 100644
index 0000000..e69de29

There is no content, so there is no ---/+++ pair and no hunk — the header simply ends at the index line. git_patch_from_buffer / git_diff_from_buffer reject this valid diff.

Why it happens

In parse_header_git() the header state machine reaches STATE_INDEX after the index line but has no accepting transition out of it when the header ends there. STATE_INDEX is not treated as a terminal state, and a following diff --git (the multi-file case, where an empty-file add is followed by another file) is not recognized from STATE_INDEX either, so parsing errors out.

#5248 previously handled empty files in git format-patch output, which terminate with a -- signature line (the { "-- ", STATE_INDEX, ... } transition). Plain git diff output has no -- line, so that path doesn't apply and the header ends directly at index — the case this change covers.

Fix

Two additive changes mirroring the existing STATE_END handling:

1. Accept STATE_INDEX as a second terminal state (for a header that ends at end-of-buffer):

   if (state != STATE_END && state != STATE_INDEX) {

2. Recognize a new diff --git header beginning from STATE_INDEX (for the multi-file case), mirroring the existing next-patch row for STATE_END:

   { "diff --git ", STATE_INDEX, 0, NULL },

A non-empty file diff still transitions out of STATE_INDEX via the --- row, so real modifications are unaffected. A truncated modification (bare index with no hunks) is still rejected by the existing check_patch() "patch with no hunks" guard, matching git apply.

Tests

Adds parser tests in tests/libgit2/patch/parse.c for an empty-file add (both at end-of-buffer and followed by a second file in a multi-file patch), reproducing the issue's exact input.

Fixes #6778.