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>