Try to fix gempush GH workflow

Got error: "fatal: ambiguous argument 'HEAD..HEAD^': unknown revision
or path not in the working tree."

Also, make gempush-if-failed script fail on error.

.github/workflows/gempush.yml

@@ -12,6 +12,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@master
+      with:
+        fetch-depth: 2
     - name: Set up Ruby 2.7
       uses: ruby/setup-ruby@v1
       with:

bin/gempush-if-changed

@@ -1,6 +1,7 @@
 #!/bin/bash
 
 set -x
+set -euo pipefail
 if git diff --name-only HEAD..HEAD^ | egrep -q '^VERSION$'; then
   # The VERSION file changed in the last commit, build the gem and push
   gem build *.gemspec