Improve .VERSION for git export-subst

We now have a correct abbreviated commit, a commit date and a git
decribe. For example:

refs=HEAD -> fix-license-dump
commit=5ccc92e69cffb503e9bedc7fce5a1dbb0fd851da
abbrev_commit=5ccc92e69c
committer_date=2023-01-16
git_describe=v31.2.3-328-g5ccc92e69c

Signed-off-by: Philippe Ombredanne <pombredanne@nexb.com>

Author: pombredanne

.VERSION

@@ -1,3 +1,5 @@
 refs=$Format:%D$
 commit=$Format:%H$
-abbrev_commit=$Format:%H$
+abbrev_commit=$Format:%h$
+committer_date=$Format:%cs$
+git_describe=$Format:%(describe)$