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>

contrib/completion/git-completion.bash

@@ -197,11 +197,16 @@ fi
 
 __gitcompadd ()
 {
-	COMPREPLY=($(compgen -W "$1" -P "$2" -S "$4" -- "$3"))
+	local i=0
+	for x in $1; do
+		if [[ "$x" == "$3"* ]]; then
+			COMPREPLY[i++]="$2$x$4"
+		fi
+	done
 }
 
-# Generates completion reply with compgen, appending a space to possible
-# completion words, if necessary.
+# Generates completion reply, appending a space to possible completion words,
+# if necessary.
 # It accepts 1 to 4 arguments:
 # 1: List of possible completion words.
 # 2: A prefix to be added to each possible completion word (optional).
@@ -221,8 +226,8 @@ __gitcomp ()
 	esac
 }
 
-# Generates completion reply with compgen from newline-separated possible
-# completion words by appending a space to all of them.
+# Generates completion reply from newline-separated possible completion words
+# by appending a space to all of them.
 # It accepts 1 to 4 arguments:
 # 1: List of possible completion words, separated by a single newline.
 # 2: A prefix to be added to each possible completion word (optional).

t/t9902-completion.sh

@@ -165,7 +165,7 @@ test_expect_success '__gitcomp - suffix' '
 	EOF
 '
 
-test_expect_failure '__gitcomp - doesnt fail because of invalid variable name' '
+test_expect_success '__gitcomp - doesnt fail because of invalid variable name' '
 	__gitcomp "$invalid_variable_name"
 '
 
@@ -204,7 +204,7 @@ test_expect_success '__gitcomp_nl - no suffix' '
 	EOF
 '
 
-test_expect_failure '__gitcomp_nl - doesnt fail because of invalid variable name' '
+test_expect_success '__gitcomp_nl - doesnt fail because of invalid variable name' '
 	__gitcomp_nl "$invalid_variable_name"
 '
 
@@ -332,7 +332,7 @@ test_expect_success 'complete tree filename with spaces' '
 	EOF
 '
 
-test_expect_failure 'complete tree filename with metacharacters' '
+test_expect_success 'complete tree filename with metacharacters' '
 	echo content >"name with \${meta}" &&
 	git add . &&
 	git commit -m meta &&