After allowing my MacBook Air to upgrade itself to OSX 10.15 Catalana, git commands throw an error:
$ git status xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun
This is because OSX updates remove the developer tools. You need to re-install them (and agree the license again).
$ xcode-select --install xcode-select: note: install requested for command line developer tools
I am grateful to the contributors on Stack Exchange for the solution to this problem.