Fetch the full history of master before diff-ing it against HEAD

The `Run tests` jobs were broken with 1ae4126
The problem is that the project uses `fetch-depth: 1` (the default) and
thus has no branch history.
Using `fetch-depth: 0` would fetch the whole history for **all**
branches and tags, which would be unnecessary.

Refs:
- actions/checkout#296
- actions/checkout#301

Signed-off-by: Martin Tzvetanov Grigorov <mgrigorov@apache.org>
(cherry picked from commit e3c24c6)

.github/workflows/GithubActionTests.yml

@@ -42,6 +42,7 @@ jobs:
         run: |
           eval "$(conda shell.bash hook)"
           conda activate bioconda
+          git fetch origin master:refs/remotes/origin/master
           if git diff --name-only origin/master...HEAD | grep -vE ^docs; then
               py.test --durations=0 test/ -v --log-level=DEBUG --tb=native -m '${{ matrix.py_test_marker }}'
           else
@@ -70,6 +71,7 @@ jobs:
         run: |
           eval "$(conda shell.bash hook)"
           conda activate bioconda
+          git fetch origin master:refs/remotes/origin/master
           if git diff --name-only origin/master...HEAD | grep -vE ^docs; then
               py.test --durations=0 test/ -v --log-level=DEBUG -k "not docker" --tb=native
           else