Fix bad behaviour when running post-review within a git-submodule with recent git versions (issue #2526)
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On recent version of git, .git is not stored in the worktree of the submodule anymore but as as a subdirectory of the top project
$ git rev-parse --git-dir
/home/nmorey/workspace/git/SigmaCToolchain/.git/modules/scdiag
As post-review tries to reach the top directory by chdir to dirname(git_dir)
we end-up into the git directory of the top project so the diff fails.
The idea is to use the --show-toplevel option of git-rev-parse instead (when it exists).
This patch fixes the issue for me