perf: do allow GIT_PERF_* to be overridden again

A common way to run Git's performance benchmarks on repositories other
than Git's own repository (which is not exactly large when compared to
actually large repositories) is to run them like this:

	GIT_PERF_LARGE_REPO=/path/to/my/large/repo \
	./p1234-*.sh -ivx

Contrary to developers' common expectations, this failed to work when
Git was built with a different `GIT_PERF_LARGE_REPO` value specified at
build time: That build-time option would have been written to the
`GIT-BUILD-OPTIONS` file, which in turn would have been sourced by
`test-lib.sh`, which in turn would have been sourced by `perf-lib.sh`,
which in turn would have been sourced by the perf test script,
_overriding_ the environment variable specified in the way illustrated
above.

Since perf tests are not run as part of the build, this most likely
unintended behavior was not caught and certainly not fixed, as the
`GIT_PERF_*` values would have been empty at build-time.

However, in 4638e88 (Makefile: use common template for
GIT-BUILD-OPTIONS, 2024-12-06), a subtle change of behavior was
introduced: Whereas before, a couple of build-time options (the
`GIT_PERF_*` ones included) were written to `GIT-BUILD-OPTIONS` only
when their values were non-empty. With this commit, they are also
written when they are empty.

The consequence is that above-mentioned way to run the perf tests will
not only fail to pick up the desired `GIT_PERF_*` settings when they
were specified differently while building Git, instead the desired
settings will be only respected when specified _while building_ Git.

Let's work around the original issue, i.e. let `GIT_PERF_*` environment
variables override what is recorded in `GIT-BUILD-OPTIONS`.

Note that this is just the tip of the iceberg, there are a couple of
`GIT_TEST_*` options that may want a similar fix in `test-lib.sh`. Due
to time constraints on my side, this here patch focuses exclusively on
the `GIT_PERF_*` settings.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

t/perf/perf-lib.sh

@@ -25,7 +25,19 @@ TEST_OUTPUT_DIRECTORY=$(pwd)
 TEST_NO_CREATE_REPO=t
 TEST_NO_MALLOC_CHECK=t
 
+# GIT-BUILD-OPTIONS, sourced by test-lib.sh, overwrites the `GIT_PERF_*`
+# values that are set by the user (if any). Let's stash them away as
+# `eval`-able assignments.
+git_perf_settings="$(env |
+	sed -n "/^GIT_PERF_/{
+		# escape all single-quotes in the value
+		s/'/'\\\\''/g
+		# turn this into an eval-able assignment
+		s/^\\([^=]*=\\)\\(.*\\)/\\1'\\2'/p
+	}")"
+
 . ../test-lib.sh
+eval "$git_perf_settings"
 
 unset GIT_CONFIG_NOSYSTEM
 GIT_CONFIG_SYSTEM="$TEST_DIRECTORY/perf/config"