completion: get rid of compgen

completion: get rid of compgen

The functionality we use from compgen is not much, we can do the same
manually, with drastic improvements in speed, especially when dealing
with only a few words.

This patch also has the sideffect that brekage reported by Jeroen Meijer
and SZEDER Gábor gets fixed because we no longer expand the resulting
words.

Here are some numbers filtering N amount of words:

  == 1 ==
  original: 0.002s
  new: 0.000s
  == 10 ==
  original: 0.002s
  new: 0.000s
  == 100 ==
  original: 0.003s
  new: 0.002s
  == 1000 ==
  original: 0.012s
  new: 0.011s
  == 10000 ==
  original: 0.056s
  new: 0.066s
  == 100000 ==
  original: 2.669s
  new: 0.622s

If the results are not narrowed:

  == 1 ==
  original: 0.002s
  new: 0.000s
  == 10 ==
  original: 0.002s
  new: 0.001s
  == 100 ==
  original: 0.004s
  new: 0.004s
  == 1000 ==
  original: 0.020s
  new: 0.015s
  == 10000 ==
  original: 0.101s
  new: 0.355s
  == 100000 ==
  original: 2.850s
  new: 31.941s

So, unless 'git checkout <tab>' usually gives you more than 10000
results, you'll get an improvement :)

Other possible solutions perform better after 1000 words, but worst if
less than that:

  COMPREPLY=($(awk -v cur="$3" -v pre="$2" -v suf="$4"
	'$0 ~ cur { print pre$0suf }' <<< "$1" ))

  COMPREPLY=($(printf -- "$2%s$4\n" $1 | grep "^$2$3"))

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>