pgsql: Reset reindex-in-progress state before reverifying an exclusion

Reset reindex-in-progress state before reverifying an exclusion constraint.

This avoids an Assert failure when we try to use ordinary index fetches
while checking for exclusion conflicts.  Per report from Noah Misch.

No need for back-patch because the Assert wasn't there before 9.1.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/dccfb72892acabd25568539ec882cc44c57c25bd

Modified Files
--------------
src/backend/catalog/index.c                |   64 +++++++++++++++++++--------
src/test/regress/input/constraints.source  |    3 +
src/test/regress/output/constraints.source |    2 +
3 files changed, 50 insertions(+), 19 deletions(-)