pgsql: Preserve information on use of git-external-diff

Preserve information on use of git-external-diff

Now that the Working with git wiki page no longer suggests producing
context diffs, we should preserve the information on how to use
git-external-diff for those people who want to view context format
diffs. The most obvious place is in the script itself, so that's what's
done here.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4431c94c3606ab54594b15ad89c7e71e46b4fb6a

Modified Files
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src/tools/git-external-diff | 35 +++++++++++++++++++++++++++++++++++
1 file changed, 35 insertions(+)