completion: let 'for-each-ref' strip the remote name from remote branches

The code listing unique remote branches for 'git checkout's tracking
DWIMery uses a shell parameter expansion in a loop iterating over each
listed ref to remove the remote's name from the remote branches, i.e.
the leading path component from the short ref.  When listing refs from
a configured remote repository, '| sed s///' is used for the same
purpose.

Let 'git for-each-ref' strip one more leading path component from the
refs, i.e. use the format 'refname:strip=3' instead of '=2', making
that parameter expansion and 'sed' execution unnecessary.

This speeds up refs completion for 'git checkout'.  Uniquely
completing a branch for 'git checkout maste<TAB>' in a repo with 100k
remote branches, all packed, best of five:

  On Linux, near the beginning of this series, for reference:

    $ time __git_complete_refs --cur=maste --track

    real    0m8.185s
    user    0m6.896s
    sys     0m1.616s

  Before this patch:

    real    0m2.714s
    user    0m2.344s
    sys     0m0.436s

  After:

    real    0m1.993s
    user    0m1.740s
    sys     0m0.304s

  On Windows, near the beginning:

    real    1m8.421s
    user    0m7.591s
    sys     0m3.557s

  Before this patch:

    real    0m8.191s
    user    0m4.638s
    sys     0m2.918s

  After:

    real    0m6.187s
    user    0m3.358s
    sys     0m2.121s

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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