tests: fix spurious failure of 'get-sysconf.test'

tests: fix spurious failure of 'get-sysconf.test'

* tests/get-sysconf.test: Do not assume that a ChangeLog file exists
in $(srcdir): now that the ChangeLog is autogenerated and not under
version control anymore, this is not necessary true.  Instead, if we
are running from a git checkout, use "git log" to get information on
the version of the automake snapshot being tested (and fall back on
extracting the first ChangeLog entry otherwise).

Reported by Jim Meyering in automake bug#10418.