bitcoin-cli doesn't need this. How come ?

Also, multiline stings are great.

This doesn't need to be in its own function. It just cause the flow to jump back and forth for no good reasons.

If you use the python requests module, the whole thing becomes:

import requests
response = requests.get(url)

if response.status_code != requests.codes.ok:
    print("curl error status: {}".format(response.status_code))
    exit(1)

return response.json()

This is more portable but also adds a new dependency, so I leave that up to you.
Link to the python requests documentation.

If the user calls update-chainparams.py -t -p 8332 it ends up changing the port silently.
You can remove the default port in argparse and if not set override it to 8332 or 18332 depending on testnet.

If the block is not found curl would return status 0, and this will crash.

The pythonic way:
chainParamsClass = 'TestNetParams' if args.testnet else 'MainParams'

\w is not exactly what you want here. The file being utf-8 it can match a lot of characters, and even in ASCII it also includes the underscore char. I think [a-zA-Z0-9] is what you want here, maybe even only the lowercase chars.

This should at least run with check-all . Please check how other tests are setup and do the same.

I don't think this is a good solution:

• the script is not run with Ctest, and does not display as part of the test suite
• this is limited to script with no arguments
• there is no dependency, so modifying the script won't regenerate the target

I suggest you use the same solution than test_runner.py or bitcoin-util-test.py (look at the end of test/CMakeLists.txt)

Why don't you want to catch exceptions from this code ?

git diff -- :/src/chainparams.cpp, otherwise I get:

fatal: ambiguous argument ':/src/chainparams.cpp': both revision and filename
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

err is not used, you can remove it

git checkout HEAD -- :/src/chainparams.cpp

Dito, err is not used

Note to reviewers: this catches the case where the result is empty (-rest=1 is not set).

Dito, err is not used.

This is a sign you wire a ton of dependencies in. This makes your code fragile. Why does the code who fetch from the node and deduce infos would depend on the git setup of the project?

You need to read your own code as you go. It is telling you what you need to do - or at least what's wrong with hat you are doing and need addressing. This has no reason to be manipulating source control. For instance, if the file is moved, there are no reason for this test to break because the path has been updated everywhere but here.