git: fix branch name during diffing (#8795)
git: fix branch name during diffing (#8795)

Fixes various early exits atm:
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https://storage.cloud.google.com/git-logs.clusterfuzz-external.appspot.com/libFuzzer_git_fuzz-cmd-diff/libfuzzer_asan_git/2022-10-17/05%3A05%3A41%3A777284.log
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https://storage.cloud.google.com/git-logs.clusterfuzz-external.appspot.com/libFuzzer_git_fuzz-cmd-diff/libfuzzer_asan_git/2022-10-17/05%3A05%3A36%3A470438.log
google/oss-fuzz,google/oss-fuzz,google/oss-fuzz,google/oss-fuzz,google/oss-fuzz,google/oss-fuzz,google/oss-fuzz,google/oss-fuzz,google/oss-fuzz,google/oss-fuzz,google/oss-fuzz