git describe compares the index with the working tree when (and only
when) it is run with the "--dirty" flag. This is done by the
run_diff_index() function. The function has been made aware of the
sparse-index in the series that led to 8d2c3732 (Merge branch
'ld/sparse-diff-blame', 2021-12-21). Hence we can just set the
requires-full-index to false for "describe".
Performance metrics
Test HEAD~1 HEAD
-------------------------------------------------------------------------------------------------
2000.2: git describe --dirty (full-v3) 0.08(0.09+0.01) 0.08(0.06+0.03) +0.0%
2000.3: git describe --dirty (full-v4) 0.09(0.07+0.03) 0.08(0.05+0.04) -11.1%
2000.4: git describe --dirty (sparse-v3) 0.88(0.82+0.06) 0.02(0.01+0.05) -97.7%
2000.5: git describe --dirty (sparse-v4) 0.68(0.60+0.08) 0.02(0.02+0.04) -97.1%
2000.6: echo >>new && git describe --dirty (full-v3) 0.08(0.04+0.05) 0.08(0.05+0.04) +0.0%
2000.7: echo >>new && git describe --dirty (full-v4) 0.08(0.07+0.03) 0.08(0.05+0.04) +0.0%
2000.8: echo >>new && git describe --dirty (sparse-v3) 0.75(0.69+0.07) 0.02(0.03+0.03) -97.3%
2000.9: echo >>new && git describe --dirty (sparse-v4) 0.81(0.73+0.09) 0.02(0.01+0.05) -97.5%
Signed-off-by: Raghul Nanth A <nanth.raghul@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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