create-git-repo.sh: fix amend error if there's no branch checked out

In this case, `git commit --amend` fails.

The state of a .git repo existing without a branch being checked out
can be reached through the following sequence of events (which I just
seem to have triggered):
1. The script calls `git init`
2. The user interrupts the script before `git commit` is finished

test/lib/create-git-repo.sh

@@ -4,7 +4,7 @@
   cd "$scriptDir/.."
   amend=--amend
 
-  if [[ ! -e .git ]]; then
+  if [[ ! -e .git ]] || ! git rev-parse HEAD 2>/dev/null; then
     git init
     amend=
   fi