Add reference fallback argument

and have it default to `HEAD`
so that the same behavior is kept
while being robust to git rebasing

users can set both reference revision and fallback to the same thing
if they would like the command to error for unresolvable references

colcon_cache/task/lock/git.py

@@ -51,9 +51,20 @@ def add_arguments(self, *, parser):  # noqa: D102
             '--git-reference-revision',
             type=str, default=None,
             help=(
-                'Optional choose revision used as a reference. If unset, '
+                'Optionally specify revision used as a reference. If unset, '
                 'the reference from the previous lockfile will be reused. '
-                'If nether provide references, revision defaults to HEAD.'
+                'If nether provide references, the fallback will be used.'
+                ' View docs for info: https://git-scm.com/docs/gitrevisions'
+                )
+            )
+
+        parser.add_argument(
+            '--git-reference-fallback',
+            type=str, default='HEAD',
+            help=(
+                'Override fallback revision used as a reference. If nether '
+                'the user and the previous lockfile specify a revision, or '
+                'if reference is unresolvable, this fallback will be used.'
                 ' View docs for info: https://git-scm.com/docs/gitrevisions'
                 )
             )
@@ -91,13 +102,13 @@ async def lock(self, *, additional_hooks=None):  # noqa: D102
     def compute_current_checksums(self, args, lockfile):  # noqa: D102
         repo = Repo(args.path, search_parent_directories=True)
 
-        if args.git_reference_revision is None:
-            reference_commit = repo.commit()
-        else:
+        reference_commit = repo.commit(args.git_reference_fallback)
+        if args.git_reference_revision:
             try:
                 reference_commit = repo.commit(args.git_reference_revision)
             except ValueError:
-                reference_commit = repo.commit()
+                pass
+
         lockfile.metadata['reference_revision'] = reference_commit.hexsha
 
         diff_args = []