[geary] Add missing build aux file for obtaining the git version.

[Michael Gratton <mjog src gnome org>]

commit eebe498a3e8a00b8a9f8346e7ec61a8e2ead83a4
Author: Michael James Gratton <mike vee net>
Date:   Fri Jan 26 14:39:49 2018 +1030

    Add missing build aux file for obtaining the git version.
    
    This is needed to make commit e8e7139 work for people that are not me.

 build-aux/git_version.py |   16 ++++++++++++++++
 1 files changed, 16 insertions(+), 0 deletions(-)
---
diff --git a/build-aux/git_version.py b/build-aux/git_version.py
new file mode 100755
index 0000000..e0c3daa
--- /dev/null
+++ b/build-aux/git_version.py
@@ -0,0 +1,16 @@
+#!/usr/bin/env python3
+
+import re
+import sys
+import subprocess
+
+try:
+  version = subprocess.check_output(
+    ['git', 'describe', '--all', '--long', '--dirty'],
+    stderr=subprocess.DEVNULL
+  )
+except:
+  sys.exit(1)
+
+version = str(version, encoding='UTF-8').strip()
+print(re.sub(r'^heads\/(.*)-0-(g.*)$', r'\1~\2', version))