libgit2: set default branch for head to main (#86224)
libgit2: set default branch for head to main (#86224)

It uses `main` as a default branch, see: https://github.com/libgit2/libgit2/branches
zyedidia/homebrew-core,mfikes/homebrew-core,cblecker/homebrew-core,Moisan/homebrew-core,uyjulian/homebrew-core,adam-moss/homebrew-core,lemzwerg/homebrew-core,zyedidia/homebrew-core,filcab/homebrew-core,stgraber/homebrew-core,sjackman/homebrew-core,nandub/homebrew-core,Homebrew/homebrew-core,Homebrew/homebrew-core,mfikes/homebrew-core,filcab/homebrew-core,uyjulian/homebrew-core,chrisfinazzo/homebrew-core,makigumo/homebrew-core,adam-moss/homebrew-core,chrisfinazzo/homebrew-core,stgraber/homebrew-core,makigumo/homebrew-core,nbari/homebrew-core,Moisan/homebrew-core,cblecker/homebrew-core,nbari/homebrew-core,sjackman/homebrew-core,lemzwerg/homebrew-core,nandub/homebrew-core