Fix parsing .git_archival.txt for archives made from tagged commits.

Given .git_archival.txt content like this:

    node: 3634907645428b9542cfb4fd1ceef08f14526cb8
    node-date: 2022-10-13T20:38:09-06:00
    describe-name: v0.0.5
    ref-names: HEAD -> main, tag: v0.0.5

we would previously fail with an exception like this:

    $ python3 -m setuptools_scm
    Traceback (most recent call last):
      [..snip..]
      File ".../setuptools_scm/git.py", line 312, in parse_archival
        return archival_to_version(data, config=config)
      File ".../setuptools_scm/git.py", line 286, in archival_to_version
        tag, number, node, _ = _git_parse_describe(archival_describe)
      File ".../setuptools_scm/git.py", line 245, in _git_parse_describe
        tag, number, node = describe_output.rsplit("-", 2)
    ValueError: not enough values to unpack (expected 3, got 1)

The problem is that the describe-name field is "v0.0.5" instead of
looking like "v0.0.5-0-g3634907".

When we invoke git-describe ourselves, we specify the --long option, but
the format-strings implemented by git-archive do not currently give us
any way to force long-output in a .git_archival.txt file.

This change detects this condition and returns None for the missing
fields.

Author: dlitz

src/setuptools_scm/git.py

@@ -232,18 +232,27 @@ 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")