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

avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla