Fix #759: Update .git_archival.txt templates to more closely match how we invoke git-describe

This covers `--tags` and `--match '*[0-9]*'` (and `--dirty` is unnecessary).
There is unfortunately no format-string equivalent to git-describe's
`--long` option,

Archives made from this repository after this change should now get
versioned correctly.

Fixes: #759

Author: dlitz

.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: $Format:%D$