40817: __git_recent_branches: Retrieve less data, but faster.

By replacing the --grep-reflog=needle argument with a ${(M)...:#needle} filter,
we retrieve less data from the reflog, and consequently run (on my test cases)
16% to 40% faster.  The trade-off is that we retrieve less data: instead of
retrieving the 1000 most recent 'checkout' operations, we retrieve the most
recent 1000 operations, which would include fewer than 1000 checkout operations.

Also change [[:xdigit:]] to [0-9a-f] since it's faster, however, the absolute
gain from this is minor compared to the cost of 'git reflog'.

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