t/perf/run: preserve GIT_PERF_* from environment

If you run:

  GIT_PERF_LARGE_REPO=/some/path ./p1006-cat-file.sh

it will use the repo in /some/path. But if you use the "run" helper
script to aggregate and compare results, like this:

  GIT_PERF_LARGE_REPO=/some/path ./run HEAD^ HEAD p1006-cat-file.sh

it will ignore that variable. This is because the presence of the
LARGE_REPO variable in GIT-BUILD-OPTIONS overrides what's in the
environment. This started with 4638e8806e (Makefile: use common template
for GIT-BUILD-OPTIONS, 2024-12-06), which now writes even empty
variables (though arguably it was wrong even before with a non-empty
value, as we generally prefer the environment to take precedence over
on-disk config).

We had the same problem in perf-lib.sh itself, and we hacked around it
with 32b74b9809 (perf: do allow `GIT_PERF_*` to be overridden again,
2025-04-04). That's what lets the direct invocation of "./p1006" work
above.

And in fact that was sufficient for "./run", too, until it started
loading GIT-BUILD-OPTIONS itself in 5756ccd181 (t/perf: fix benchmarks
with out-of-tree builds, 2025-04-28). Now it has the same problem: it
clobbers any incoming GIT_PERF options from the environment.

We can use the same hack here in the "run" script. It's quite ugly, but
it's just short enough that I don't think it's worth trying to factor it
out into a common shell library.

In the long run, we might consider teaching GIT-BUILD-OPTIONS to be more
gentle in overwriting existing entries. There are probably other
GIT_TEST_* variables which would need the same treatment. And if and
when we come up with a more complete solution, we can use it in both
spots.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/run

@@ -204,8 +204,18 @@ run_subsection () {
 get_var_from_env_or_config "GIT_PERF_CODESPEED_OUTPUT" "perf" "codespeedOutput" "--bool"
 get_var_from_env_or_config "GIT_PERF_SEND_TO_CODESPEED" "perf" "sendToCodespeed"
 
+# Preserve GIT_PERF settings from the environment when loading
+# GIT-BUILD-OPTIONS; see the similar hack in perf-lib.sh.
+git_perf_settings="$(env |
+        sed -n "/^GIT_PERF_/{
+                # escape all single-quotes in the value
+                s/'/'\\\\''/g
+                # turn this into an eval-able assignment
+                s/^\\([^=]*=\\)\\(.*\\)/\\1'\\2'/p
+        }")"
 cd "$(dirname $0)"
 . ../../GIT-BUILD-OPTIONS
+eval "$git_perf_settings"
 
 if test -n "$TEST_OUTPUT_DIRECTORY"
 then