feat: Checkout tags as well as branches when cloning into scratch

[tpoliaw]

The branch field in the config file can now be set to the value of a
tag. While it could be confusing to use 'branch' for both, it mirrors
the behaviour of the --branch flag when cloning repositories with the
git CLI.

In a slight change in behaviour, the branch field is now only used when
first cloning a repository - if it already exists in the scratch area it
will be left in the same state. This is to avoid the ambiguity of how
tags should behave when the repo has been modified since being cloned.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.19%. Comparing base (65b7e75) to head (d0178b1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1482      +/-   ##
==========================================
- Coverage   95.21%   95.19%   -0.02%     
==========================================
  Files          43       43              
  Lines        3132     3123       -9     
==========================================
- Hits         2982     2973       -9     
  Misses        150      150              

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