homebrewをインストールする
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"
Cloning into '/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core'...
fatal: unable to access 'https://github.com/Homebrew/homebrew-core/': LibreSSL SSL_read: SSL_ERROR_SYSCALL, errno 54
Error: Failure while executing; `git clone https://github.com/Homebrew/homebrew-core /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core` exited with 128.
Error: Failure while executing; `/usr/local/bin/brew tap homebrew/core` exited with 1.
Failed during: /usr/local/bin/brew update --force
...ん？
brew doctor してみる
$ brew doctor
==> Tapping homebrew/core
Cloning into '/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core'...
remote: Enumerating objects: 40, done.
remote: Counting objects: 100% (40/40), done.
remote: Compressing objects: 100% (27/27), done.
error: RPC failed; curl 18 transfer closed with outstanding read data remaining
fatal: the remote end hung up unexpectedly
fatal: early EOF
fatal: index-pack failed
Error: Failure while executing; `git clone https://github.com/Homebrew/homebrew-core /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core` exited with 128.
Error: Failure while executing; `/usr/local/bin/brew tap homebrew/core` exited with 1.
やっぱり普通にcloneに失敗してるっぽい？
解決
$ git clone https://github.com/Homebrew/homebrew-core /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core --depth=1
Cloning into '/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core'...
remote: Enumerating objects: 5370, done.
remote: Counting objects: 100% (5370/5370), done.
remote: Compressing objects: 100% (5151/5151), done.
remote: Total 5370 (delta 53), reused 355 (delta 10), pack-reused 0
Receiving objects: 100% (5370/5370), 4.43 MiB | 25.00 KiB/s, done.
Resolving deltas: 100% (53/53), done.
Updating files: 100% (5390/5390), done.
$ brew doctor
Your system is ready to brew.
OK!
macOS Catalina 10.15.4
git version 2.24.2 (Apple Git-127)