pod repo update should unset GIT_DIR before running git operations

[lilyball]

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What did you do?

I updated my Podfile and ran pod update someDep from within a git rebase operation, which triggered a pod repo update.

What did you expect to happen?

This should have updated the cocoapods repo.

What happened instead?

Because I was in the middle of a git rebase, the GIT_DIR environment variable was set. This made CocoaPods's git fetch operation actually apply the operation to my local repo instead of the cocoapods spec repo.

Notes

The GIT_DIR environment variable should be unset, but there's probably others that need to be unset too.

[segiddins]

Should be fairly easy to do, just by wrapping the git operations done in the source manager in a block that saves the ENV, unsets the git stuff, runs the fetch, and then restores