Crash on git log with -L and -G on file rename

[Matthew Hughes <matthewhughes934@gmail.com>]

Hi,

I hit a crash (assertion error) running `git log` with both `-L` and `-G` under
certain conditions. I've created script to reproduce the behaviour in a fresh
git repo:

    #!/usr/bin/env bash

    set -o errexit

    git init .
    # Note: example is .rs file, but it should work with anything that -L understands
    echo "fn my_func() {}" > file.rs
    # 1. file named 'file.rs'
    git add file.rs
    git commit --message 'Add the file'

    # 2. separate branch with the file renamed
    git checkout -b some-branch
    git mv file.rs new_file.rs
    git commit --message 'Move the file'

    git checkout -
    git commit --allow-empty --message 'Some extra commit so we get a merge commit'
    # 3. merge: one parent has file.rs, the other has new_file.rs
    git merge --no-edit some-branch

    # 4. post merge, move the file back
    git mv new_file.rs file.rs
    git commit --message 'Move the file back'

    # 5. things go BOOM
    git log -L:my_func:file.rs -G '.'

I'm not sure if _every_ step in that script is necessary, but it's the simplest
setup I could figure out to trigger the crash. Running that script I hit the
error:

    git: line-log.c:1056: process_diff_filepair: Assertion `pair->two->oid_valid' failed.
    Aborted                    (core dumped) git log -L:my_func:file.rs -G '.'

The backtrace shows that the failed assertion occurs under
`process_ranges_merge_commit`, so maybe there's an issue with the file being
renamed on both sides of the merge?

The crash requires both flags to trigger, remove either and it will run fine.

I've tested the above on the `git` from my system package manager on Arch
Linux: git version 2.53.0, and one built from source at
2cc71917514657b93014134350864f4849edfc83 (the version of 'master' checked out
on my machine at the time). I don't think reproduction relies on any specific
config since I've had it trigger with both `GIT_CONFIG_GLOBAL` and
`GIT_CONFIG_SYSTEM` set to `/dev/null`

Just for reference, I originally triggered the bug in the `rustfmt` repo[1]
(checked-out at cebab3e99259be82ff069e5ae89e91855d79e534) running:

    git log -G offset_left -L:format_trait:src/items.rs

Link: github.com/rust-lang/rustfmt [1]