tests: avoid spurious failures in get-sysconf.test

tests: avoid spurious failures in get-sysconf.test

This change fixes the last bit of automake bug#10866.

* tests/get-sysconf.test: When Homebrew spawns a build, even if
from a project's VCS, it stages the files in a new temporary
directory, instead of building directly from a VCS checkout.
This behaviour was causing a spurious failure in this test, which
expected to find either a ChangeLog file or a .git directory in
the source directory (the former happening when the test was run
from a distribution tarball, the latter when it was run from a
git checkout).  The Homebrew issue shows that these expectations
are not truly warranted, so relax the test to just give a warning,
not a failure, in case they do not hold.  Since we are at it, fix
an unrelated weakness in the displaying of the git log, which
could have failed when the builddir was not a subdirectory of the
srcdir.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>