[clang] [git-clang-format] Don't format the line preceding a deletion (PR #215946)

[Fangrui Song via cfe-commits]

MaskRay wrote:

Tested a few scenarios. git-clang-format should now match clang-format-diff.py. 

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Confirmed — but with one wrinkle worth knowing before review.

The behavioral claim holds. I fed identical scenarios to both tools and compared:

┌───────────────────────┬─────────────────┬──────────────────────┬──────────────────────────┐
│       Scenario        │   Hunk header   │ clang-format-diff.py │ git-clang-format (fixed) │
├───────────────────────┼─────────────────┼──────────────────────┼──────────────────────────┤
│ Pure deletion         │ @@ -3,3 +2,0 @@ │ no changes           │ no changes               │
├───────────────────────┼─────────────────┼──────────────────────┼──────────────────────────┤
│ Modification          │ @@ -6 +6 @@     │ changes proposed     │ changes proposed         │
├───────────────────────┼─────────────────┼──────────────────────┼──────────────────────────┤
│ Insertion             │ @@ -7,0 +8 @@   │ changes proposed     │ changes proposed         │
├───────────────────────┼─────────────────┼──────────────────────┼──────────────────────────┤
│ Leading-line deletion │ @@ -1,2 +0,0 @@ │ no changes           │ no changes               │
└───────────────────────┴─────────────────┴──────────────────────┴──────────────────────────┘

They agree everywhere. And the divergence the patch closes is real, not imagined: on the pure-deletion case the unpatched git-clang-format proposes reflowing the untouched neighbor while clang-format-diff.py proposes nothing, on byte-identical input.

```

https://github.com/llvm/llvm-project/pull/215946