Fix "not enough values to unpack" when parsing .git_archival.txt from a tagged commit; also fix #759

.git_archival.txt

@@ -1,4 +1,4 @@
 node: $Format:%H$
 node-date: $Format:%cI$
-describe-name: $Format:%(describe)$
+describe-name: $Format:%(describe:tags=true,match=*[0-9]*)$
 ref-names: $Format:%D$

README.rst

@@ -316,7 +316,7 @@ and copy-paste this into it::
 
     node: $Format:%H$
     node-date: $Format:%cI$
-    describe-name: $Format:%(describe:tags=true)$
+    describe-name: $Format:%(describe:tags=true,match=*[0-9]*)$
     ref-names: $Format:%D$
 
 Create the ``.gitattributes`` file in the root directory of your repository

src/setuptools_scm/.git_archival.txt

@@ -1,4 +1,4 @@
 node: $Format:%H$
 node-date: $Format:%cI$
-describe-name: $Format:%(describe)$
+describe-name: $Format:%(describe:tags=true,match=*[0-9]*)$
 ref-names: $Format:%D$

src/setuptools_scm/git.py

@@ -232,18 +232,29 @@ def _git_parse_inner(
     )
 
 
-def _git_parse_describe(describe_output: str) -> tuple[str, int, str, bool]:
+def _git_parse_describe(
+    describe_output: str,
+) -> tuple[str, int | None, str | None, bool]:
     # 'describe_output' looks e.g. like 'v1.5.0-0-g4060507' or
     # 'v1.15.1rc1-37-g9bd1298-dirty'.
+    # It may also just be a bare tag name if this is a tagged commit and we are
+    # parsing a .git_archival.txt file.
 
     if describe_output.endswith("-dirty"):
         dirty = True
         describe_output = describe_output[:-6]
     else:
         dirty = False
 
-    tag, number, node = describe_output.rsplit("-", 2)
-    return tag, int(number), node, dirty
+    split = describe_output.rsplit("-", 2)
+    if len(split) < 3:  # probably a tagged commit
+        tag = describe_output
+        number = None
+        node = None
+    else:
+        tag, number_, node = split
+        number = int(number_)
+    return tag, number, node, dirty
 
 
 def search_parent(dirname: _t.PathT) -> GitWorkdir | None:

testing/test_git.py

@@ -523,6 +523,7 @@ def test_git_getdate_signed_commit(signed_commit_wd: WorkDir) -> None:
         ("0.0", {"node": "0" * 20}),
         ("1.2.2", {"describe-name": "release-1.2.2-0-g00000"}),
         ("1.2.2.dev0", {"ref-names": "tag: release-1.2.2.dev"}),
+        ("1.2.2", {"describe-name": "v1.2.2"}),
     ],
 )
 @pytest.mark.filterwarnings("ignore:git archive did not support describe output")