Add GitPython's standard license header comments to top level scripts

While discussing adding similar license comments to the shell scripts
introduced in PR gitpython-developers#1901, it was noticed that the shell scripts in the
repository root directory did not have such comments and suggested that
we could add them when the scripts in the `fuzzing/` directory were
updated, so this commit does just that.

See:
gitpython-developers#1901 (comment)

Author: DaveLak

Diff for: build-release.sh

@@ -1,5 +1,8 @@
 #!/bin/bash
 #
+# This file is part of GitPython and is released under the
+# 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
+#
 # This script builds a release. If run in a venv, it auto-installs its tools.
 # You may want to run "make release" instead of running this script directly.
 

Diff for: check-version.sh

@@ -1,5 +1,8 @@
 #!/bin/bash
 #
+# This file is part of GitPython and is released under the
+# 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
+#
 # This script checks if we are in a consistent state to build a new release.
 # See the release instructions in README.md for the steps to make this pass.
 # You may want to run "make release" instead of running this script directly.

Diff for: init-tests-after-clone.sh

@@ -1,4 +1,7 @@
 #!/bin/sh
+#
+# This file is part of GitPython and is released under the
+# 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
 set -eu