task template: paired timing builds both sides the same way

[ArjunS07]

Purpose

Make paired timing compare the patch, not two different builds. When a patch touches compiled sources, only the patched tree is rebuilt, and the base copy keeps the image's build.

How it works

test.sh rebuilds the patched tree (FC_REBUILD_CMD, rebuild.sh) before timing. The base copy /workspace/.fc_base was copied before that, so it still has the image's original build. If the trial's rebuild uses different flags or a different path than the image build (for example, setup.py build_ext --inplace after both pip install -e attempts fail), the timed difference includes the build difference.

lsv_measure.py now runs FC_REBUILD_CMD on the base copy at the repo path (with base_in_place(), as for the timing rounds), then on the patched tree again, before paired timing. Both sides come from the same command at the same path. A failed rebuild ends the measure with an error instead of timing two different builds. The log is rebuild_both.log.

Changes

• template/tests/lsv_measure.py: rebuild_both_sides(), called before measure_paired when FC_REBUILD_CMD is set.
  • Effect: base and patched trees are built the same way.
  • Before: numpy#21464 (in-place fallback build) showed the same shift in all 3 no-change runs as in the oracle runs. About 520 of 3,481 benchmarks were above log 0.3 and about 50 below -0.3, while the oracle patch is 14 lines in ufunc_object.c. In the survey's approved records with rebuilds, the share of no-change benchmarks with |log speedup| > 0.1 is 0.29 (numpy#21464), 0.42 (bottleneck#329) and 0.16 (bottleneck#327), against at most 0.07 for the other 20.
• tests/docker/test_lsv_paired_measure.py: the rebuild runs base first and then patched, both at the repo path, and a failed base rebuild leaves the patched tree in place.

Verification

• The 2 new tests pass. They fail without the change, because the function is missing.
• tests/docker: the 7 failures present without the change are unchanged.
• To do: re-measure numpy#21464, bottleneck#329 and bottleneck#327 with this template. The no-change share above should drop to the level of the other tasks.

Notes

• Each compiled task's trial now rebuilds twice more: once for the base and once for the patched tree again.