Capture version in archives with .git_archival.txt

The .git_archival.txt file is configured (via .gitattributes) to be
formatted at `git archive` time with the ref names associated with the
archive. When archiving a tag, the tag name(s) will appear in the
archived .git_archival.txt file as 'ref-names: tag: vX.Y, ...'.

N.B. this .git_archival.txt file is compatible with the setuptools_scm
setuptools_scm_git_archive plugin. However, that toolchain is not being
used at this time.

Signed-off-by: Peter Grayson <[email protected]>

Author: jpgrayson

.git_archival.txt

@@ -0,0 +1 @@
+ref-names: $Format:%D$

.gitattributes

@@ -0,0 +1 @@
+.git_archival.txt export-subst