BUG #6602: concurrent psql session clobbers history

The following bug has been logged on the website:

Bug reference: 6602
Logged by: Adrian Dries
Email address: i+pgbugs(at)avdd(dot)tk
PostgreSQL version: 9.1.3
Operating system: Ubuntu ppa:pitti/postgresql
Description:

A concurrent psql session will overwrite the history of the first. This
does not look good: a database tool causing me to lose my data!

# session 1

$ psql -q
postgres=# \echo first session exits first
first session exits first
postgres=# \q
$ cat .psql_history
\echo first session exits first
\q
$ psql -q
postgres=#
$ cat .psql_history
\echo second session exits second
\q
$ psql -q
postgres=# \echo first session exits second
first session exits second
postgres=# \q
$ cat .psql_history
\echo second session exits second
\q
\echo first session exits second
\q

# session 2

$ psql -q
postgres=# \echo second session exits second
second session exits second
postgres=# \q
$ cat .psql_history
\echo second session exits second
\q
$ psql -q
postgres=# \echo second session exits first
second session exits first
postgres=# \q
$ cat .psql_history
\echo second session exits second
\q
\echo second session exits first
\q