pgsql: Fix incorrect return value for pg_size_pretty(bigint)

Fix incorrect return value for pg_size_pretty(bigint)

pg_size_pretty(bigint) would return the value in bytes rather than PB
for the smallest-most bigint value. This happened due to an incorrect
assumption that the absolute value of -9223372036854775808 could be
stored inside a signed 64-bit type.

Here we fix that by instead storing that value in an unsigned 64-bit type.

This bug does exist in versions prior to 15 but the code there is
sufficiently different and the bug seems sufficiently non-critical that
it does not seem worth risking backpatching further.

Author: Joseph Koshakow <koshy44(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAAvxfHdTsMZPWEHUrZ=h3cky9Ccc3Mtx2whUHygY+ABP-mCmUw@mail.gmail.com
Backpatch-through: 15

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b181062aa5727a013c96b64476f884c992b5068d

Modified Files
--------------
src/backend/utils/adt/dbsize.c | 8 ++++++--
src/test/regress/expected/dbsize.out | 8 ++++++++
src/test/regress/sql/dbsize.sql | 4 ++++
3 files changed, 18 insertions(+), 2 deletions(-)