[pre-commit] Set default_stages to pre-commit

[pre-commit] Set default_stages to pre-commit

I realized I was seeing the newly added tclint check twice:
...
$ touch gdb/testsuite/gdb.base/foo.exp
$ git add gdb/testsuite/gdb.base/foo.exp
$ git commit -a -m foo 2>&1 | grep tclint
tclint..................................................................Passed
tclint..............................................(no files to check)Skipped
$
...

The hook is run once for stage pre-commit, and once for stage commit-msg.

Since the hook doesn't specify a stage at which it's supposed to be run, it
takes its default from default_stages, which defaults to all stages.

Fix this by setting default_stages to pre-commit:
...
$ git commit -a -m foo 2>&1 | grep tclint
tclint..................................................................Passed
$
...

The only hook sofar that needs a different stage than pre-commit is
codespell-log, and it's not affected by this change because it has an explicit
"stages: [commit-msg]" setting.

Approved-By: Tom Tromey <tom@tromey.com>