Git hangs with a ssh remote URI after 10.15.4 update
Debugging steps:


Attempted to connect with netcat:


$ nc -v gitlab.example.com 30814
Connection to gitlab.example.com port 30814 [tcp/*] succeeded!
SSH-2.0-OpenSSH_7.2
^C



Suspecting a broken ssh client, I installed openssh using brew:


$ brew install openssh
...
$ /usr/local/bin/ssh  -V
OpenSSH_8.2p1, OpenSSL 1.1.1d  10 Sep 2019

$ /usr/local/bin/ssh -p 30814 git@gitlab.example.com
Welcome to GitLab, @username!
Connection to gitlab.example.com closed.


After refreshing my shell environment to ensure that the new ssh is found in my $PATH, git operations began functioning as expected with ssh:// URIs.

Searching turned up this user submitted bug: ssh fails when using -p flag

The workaround detailed in the bug is to use the remote host's IP address instead of the DNS name. I can confirm that this worked for me:

$ /usr/bin/ssh -V
OpenSSH_8.1p1, LibreSSL 2.7.3

$ /usr/bin/ssh -p 30814 git@192.168.1.1
Welcome to GitLab, @username!
Connection to gitlab.example.com closed.


The apparent trigger for this bug is using ports greater than 8192 in conjunction with a host's DNS name.