Convert cvsignore to gitignore, and add .gitignore for build targets.

contrib/pg_archivecleanup/.gitignore

+/pg_archivecleanup

contrib/pg_buffercache/.gitignore

+/pg_buffercache.sql

contrib/pg_freespacemap/.gitignore

+/pg_freespacemap.sql

contrib/pg_standby/.gitignore

+/pg_standby

contrib/pg_stat_statements/.gitignore

+/pg_stat_statements.sql

contrib/pg_trgm/.gitignore

+/pg_trgm.sql

contrib/pg_upgrade/.gitignore

+/pg_upgrade

contrib/pgbench/.gitignore

+/pgbench

contrib/pgcrypto/.gitignore

+/pgcrypto.sql

contrib/pgrowlocks/.gitignore

+/pgrowlocks.sql

contrib/pgstattuple/.gitignore

+/pgstattuple.sql

contrib/seg/.cvsignore

-segparse.c
-segscan.c

contrib/seg/.gitignore

+/segparse.c
+/segscan.c
+/seg.sql

contrib/spi/.gitignore

+/autoinc.sql
+/insert_username.sql
+/moddatetime.sql
+/refint.sql
+/timetravel.sql

contrib/sslinfo/.gitignore

+/sslinfo.sql

contrib/tablefunc/.gitignore

+/tablefunc.sql

contrib/test_parser/.gitignore

+/test_parser.sql

contrib/tsearch2/.gitignore

+/tsearch2.sql

contrib/unaccent/.gitignore

+/unaccent.sql

contrib/vacuumlo/.gitignore

+/vacuumlo