SCYLLA-VERSION-GEN: skip updating version files when git hash unchanged

On our build system we tries to build relocatable package multiple times on
same revision of the repository, it executes ./SCYLLA-VERSION-GEN for each time.
When the build job invoked at midnight and it did not finished until 12:00AM,
first build and last build has different SCYLLA-RELEASE-FILE, since it contains
current date.
To prevent it, skip updating SCYLLA-*-FILE when git hash unchanged.

Fixes scylladb/scylla-pkg#826

SCYLLA-VERSION-GEN

@@ -19,6 +19,14 @@ else
 	SCYLLA_RELEASE=$SCYLLA_BUILD.$DATE.$GIT_COMMIT
 fi
 
+if [ -f build/SCYLLA-RELEASE-FILE ]; then
+	RELEASE_FILE=$(cat build/SCYLLA-RELEASE-FILE)
+	GIT_COMMIT_FILE=$(cat build/SCYLLA-RELEASE-FILE |cut -d . -f 3)
+	if [ "$GIT_COMMIT" = "$GIT_COMMIT_FILE" ]; then
+		exit 0
+	fi
+fi
+
 echo "$SCYLLA_VERSION-$SCYLLA_RELEASE"
 mkdir -p build
 echo "$SCYLLA_VERSION" > build/SCYLLA-VERSION-FILE