hghave: cache the result of gethgversion
hghave --test-features calls it 90 times, each one calling hg --version
which takes a tenth of a second on my workstation, adding up to about
10s win on test-hghave.t.
Fixes https://bugs.debian.org/939756
Differential Revision: https://phab.mercurial-scm.org/D8092
# See http://EditorConfig.org for the specification
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[*.t]
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