g.extension: fix extracting Git version string

[tmszi]

Describe the bug
Installing extension via g.extension module fail.

To Reproduce
Steps to reproduce the behavior:

1. Try to install some extension g.extension db.join
2. See error

in _get_version
    return float(
           ^^^^^^
ValueError: could not convert string to float: 'Git-143)'

Expected behavior
Installing extension via g.extension module should work without error message.

System description:

• Operating System: macOS
• GRASS GIS version 8.3.dev

Additional context
To be backported with #2895.

[nilason]

Mac runner fails with WARNING: The download of the commit from the GitHub API server wasn't successful, <rate limit exceeded>. Wasn't the use of git a way to circumvent this?

[tmszi]

Mac runner fails with WARNING: The download of the commit from the GitHub API server wasn't successful, <rate limit exceeded>. Wasn't the use of git a way to circumvent this?

Yes you are right, but it is another PR #2717.

[nilason]

Have not tested, but the regex looks good to me!