Update the target branch in testing to reduce the testing diff.

As title. This is to make the "files changed" that public CI has
to check smaller as it's against the release branch and not
`main`.

numba/testing/main.py

@@ -207,7 +207,8 @@ def git_diff_str(x):
         parser.add_argument('-g', '--gitdiff', dest='gitdiff', type=git_diff_str,
                             default=False, nargs='?',
                             help=('Run tests from changes made against '
-                                  'origin/main as identified by `git diff`. '
+                                  'origin/release0.56 as identified by'
+                                  '`git diff`. '
                                   'If set to "ancestor", the diff compares '
                                   'against the common ancestor.'))
         return parser
@@ -399,9 +400,9 @@ def _choose_gitdiff_tests(tests, *, use_common_ancestor=False):
     path = os.path.join('numba', 'tests')
     if use_common_ancestor:
         print(f"Git diff by common ancestor")
-        target = 'origin/main...HEAD'
+        target = 'origin/release0.56...HEAD'
     else:
-        target = 'origin/main..HEAD'
+        target = 'origin/release0.56..HEAD'
     gdiff_paths = repo.git.diff(target, path, name_only=True).split()
     # normalise the paths as they are unix style from repo.git.diff
     gdiff_paths = [os.path.normpath(x) for x in gdiff_paths]