repack: update usage to match docs

The t0450 test script verifies that the builtin usage matches the
synopsis in the documentation. Update 'git repack' to match and remove
it from the exception list.

Signed-off-by: Derrick Stolee <[email protected]>

Author: derrickstolee

t/t0450/txt-help-mismatches

@@ -44,7 +44,6 @@ rebase
 remote
 remote-ext
 remote-fd
-repack
 reset
 restore
 rev-parse