[3.6] bpo-30232: Support Git worktree in configure.ac (#1398)

* bpo-30232: Support Git worktree in configure.ac (#1391)

Don't test if .git/HEAD file exists, but only if the .git file (or
directory) exists.
(cherry picked from commit 5facdbb29169c2799c42f887cef4cd9d087b0167)

* bpo-30232: Regenerate configure (#1396)

Run autoconf.
(cherry picked from commit 9ed34a89532763cf89f5e11fffb91ef7dee29fed)
2 files changed