[COMMITTERS] pgsql: Add a "subtransaction" command to PL/Tcl.

Add a "subtransaction" command to PL/Tcl.

This allows rolling back the effects of some SPI commands without
having to fail the entire PL/Tcl function.

Victor Wagner, reviewed by Pavel Stehule

Discussion: https://postgr.es/m/20170108205750.2dab04a1@wagner.wagner.home

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b58fd4a9cab21e9d937a4e369bab31b3a5d24710

Modified Files
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doc/src/sgml/pltcl.sgml               | 112 ++++++++++++++++++++++++--
src/pl/tcl/Makefile                   |   2 +-
src/pl/tcl/expected/pltcl_subxact.out | 143 ++++++++++++++++++++++++++++++++++
src/pl/tcl/pltcl.c                    |  53 +++++++++++++
src/pl/tcl/sql/pltcl_subxact.sql      |  95 ++++++++++++++++++++++
5 files changed, 398 insertions(+), 7 deletions(-)