pgsql: Update top-level .gitignore.

Update top-level .gitignore.

Add a comment explaining our policy that we don't put excludes
for nonstandard tools into committed .gitignore files.

Also, remove the entries about libraries with .sl extensions,
since that convention was only used on now-desupported HP-UX.

Discussion: https://postgr.es/m/CAHxW8BAiyPwfXbN813GhorQozwMBs4f3DTxLkKNxiGQuJuw4Vw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d94f32d49f620fb08c1fd0e8c9345844ccd9b7c0

Modified Files
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.gitignore | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)