psql \ir filename normalization

Hi all,

Commit c7f23494c1103f87bcf1ef7cbfcd626e73edb337 editorialized a bit on
Gurjeet Singh's patch to implement \ir for psql, particularly in
process_file(). Unfortunately, it looks like it broke the common case
of loading a .SQL file in psql's working directory. Consider the
following test case:

mkdir -p /tmp/psql_test/subdir/
mkdir -p /tmp/psql_test/path2/

echo "SELECT 'hello 1';" > /tmp/psql_test/hello.sql
echo "SELECT 'hello from parent';" > /tmp/psql_test/hello_parent.sql
echo "SELECT 'hello from absolute path';" >
/tmp/psql_test/path2/absolute_path.sql
echo -e "SELECT 'hello 2';\n\ir ../hello_parent.sql\n\ir
/tmp/psql_test/path2/absolute_path.sql" >
/tmp/psql_test/subdir/hello2.sql
echo -e "\ir hello.sql\n\ir subdir/hello2.sql" > /tmp/psql_test/load.sql


If you try to load in "load.sql" from any working directory other than
/tmp/psql_test/ , you should correctly see four output statements.
However, if you:
  cd /tmp/psql_test/ && psql test -f load.sql

You will get:

psql:load.sql:1: /hello.sql: No such file or directory
psql:load.sql:2: /subdir/hello2.sql: No such file or directory

Attached is a patch which fixes this, by recycling the bit of
Gurjeet's code which used "last_slash". (I have a feeling there's a
simpler way to fix it which avoids the last_slash complications.)

Josh