completion: improve ls-files filter performance

From the output of ls-files, we remove all but the leftmost path
component and then we eliminate duplicates. We do this in a while loop,
which is a performance bottleneck when the number of iterations is large
(e.g. for 60000 files in linux.git).

$ COMP_WORDS=(git status -- ar) COMP_CWORD=3; time _git

real    0m11.876s
user    0m4.685s
sys     0m6.808s

Replacing the loop with the cut command improves performance
significantly:

$ COMP_WORDS=(git status -- ar) COMP_CWORD=3; time _git

real    0m1.372s
user    0m0.263s
sys     0m0.167s

The measurements were done with Msys2 bash, which is used by Git for
Windows.

When filtering the ls-files output we take care not to touch absolute
paths. This is redundant, because ls-files will never output absolute
paths. Remove the unnecessary operations.

The issue was reported here:
https://github.com/git-for-windows/git/issues/1533

Signed-off-by: Clemens Buchacher <drizzd@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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