Incompatibility with git-branchless #869

I use both overcommit and git-branchless. Both of these tools install git hooks. I configured overcommit to call the branchless hooks using the PostRewrite: CustomScript setting, but this causes the git branchless hook to crash.

However if I just insert

`git branchless hook post-rewrite #{ARGV}`

as the first line of overcommit's .git/hooks/post-rewrite things are fine.

There are more details here arxanas/git-branchless#1534

My question is - what is different about just hacking this line into post-rewrite vs using the CustomScript config? It seems some overcommit code that uses CustomScript changes something in the environment that breaks the git branchless call.