In sys.version and initial REPL message, list the source commit as "default"

We create an artificial Git repo to apply patches.
Python build system thinks that it's building from Git and embeds a bogus
branch name into its version string:

$ python3
Python 3.9.0b5 (heads/master-dirty:24c7f56, Aug  3 2020, 00:00:00)
[GCC 10.2.1 20200804 (Red Hat 10.2.1-2)] on linux
Type "help", "copyright", "credits" or "license" for more information.

Tell configure that Git is not installed. This reverts that part
of sys.version to "default".

python3.9.spec

@@ -17,7 +17,7 @@ URL: https://www.python.org/
 %global prerel rc1
 %global upstream_version %{general_version}%{?prerel}
 Version: %{general_version}%{?prerel:~%{prerel}}
-Release: 1%{?dist}
+Release: 2%{?dist}
 License: Python
 
 
@@ -690,6 +690,12 @@ rm configure pyconfig.h.in
 
 %build
 
+# The build process embeds version info extracted from the Git repository
+# into the Py_GetBuildInfo and sys.version strings.
+# Our Git repository is artificial, so we don't want that.
+# Tell configure to not use git.
+export HAS_GIT=not-found
+
 # Regenerate the configure script and pyconfig.h.in
 autoconf
 autoheader
@@ -1634,6 +1640,9 @@ CheckPython optimized
 # ======================================================
 
 %changelog
+* Wed Aug 12 2020 Petr Viktorin <pviktori@redhat.com> - 3.9.0~rc1-2
+- In sys.version and initial REPL message, list the source commit as "default"
+
 * Tue Aug 11 2020 Miro Hrončok <mhroncok@redhat.com> - 3.9.0~rc1-1
 - Update to 3.9.0rc1