perf-lib.sh: remove GIT_TEST_INSTALLED from perf-lib.sh

perf-lib.sh: remove GIT_TEST_INSTALLED from perf-lib.sh

Follow-up my preceding change which fixed the immediate "./run
<revisions>" regression in 0baf78e7bc ("perf-lib.sh: rely on
test-lib.sh for --tee handling", 2019-03-15) and entirely get rid of
GIT_TEST_INSTALLED from perf-lib.sh (and aggregate.perl).

As noted in that change the dance we're doing with GIT_TEST_INSTALLED
perf-lib.sh isn't necessary, but there I was doing the most minimal
set of changes to quickly fix a regression.

But it's much simpler to never deal with the "GIT_TEST_INSTALLED" we
were setting in perf-lib.sh at all. Instead the run_dirs_helper() sets
the previously inferred $PERF_RESULTS_PREFIX directly.

Setting this at the callsite that's already best positioned to
exhaustively know about all the different cases we need to handle
where PERF_RESULTS_PREFIX isn't what we want already (the empty
string) makes the most sense. In one-off cases like:

    ./run ./p0000-perf-lib-sanity.sh
    ./p0000-perf-lib-sanity.sh

We'll just do the right thing because PERF_RESULTS_PREFIX will be
empty, and test-lib.sh takes care of finding where our git is.

Any refactoring of this code needs to change both the shell code and
the Perl code in aggregate.perl, because when running e.g.:

    ./run ../../ -- <test>

The "../../" path to a relative bindir needs to be munged to a
filename containing the results, and critically aggregate.perl does
not get passed the path to those aggregations, just "../..".

Let's fix cases where aggregate.perl would print e.g. ".." in its
report output for this, and "git" for "/home/avar/g/git", i.e. it
would always pick the last element. Now'll always print the full path
instead.

This also makes the code sturdier, e.g. you can feed "../.."  to
"./run" and then an absolute path to the aggregate.perl script, as
long as the absolute path and "../.." resolved to the same directory
printing the aggregation will work.

Also simplify the "[_*]" on the RHS of "tr -c", we're trimming
everything to "_", so we don't need that.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>