Speed up bash completion loading

Since git is not used in each and every interactive xterm, it
seems best to load completion support with cold caches and then
load each needed thing lazily.  This has most of the speed
advantage of pre-generating everything at build time, without the
complication of figuring out at build time what commands will be
available at run time.

On this slow laptop, this decreases the time to load
git-completion.bash from about 500 ms to about 175 ms.

Suggested-by: Kirill Smelkov <kirr@mns.spb.ru>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Cc: Stephen Boyd <bebarino@gmail.com>
Cc: SZEDER Gábor <szeder@ira.uka.de>
Cc: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -21,13 +21,7 @@
 #    2) Added the following line to your .bashrc:
 #        source ~/.git-completion.sh
 #
-#    3) You may want to make sure the git executable is available
-#       in your PATH before this script is sourced, as some caching
-#       is performed while the script loads.  If git isn't found
-#       at source time then all lookups will be done on demand,
-#       which may be slightly slower.
-#
-#    4) Consider changing your PS1 to also show the current branch:
+#    3) Consider changing your PS1 to also show the current branch:
 #        PS1='[\u@\h \W$(__git_ps1 " (%s)")]\$ '
 #
 #       The argument to __git_ps1 will be displayed only if you
@@ -324,12 +318,8 @@ __git_remotes ()
 	done
 }
 
-__git_merge_strategies ()
+__git_list_merge_strategies ()
 {
-	if [ -n "${__git_merge_strategylist-}" ]; then
-		echo "$__git_merge_strategylist"
-		return
-	fi
 	git merge -s help 2>&1 |
 	sed -n -e '/[Aa]vailable strategies are: /,/^$/{
 		s/\.$//
@@ -339,8 +329,17 @@ __git_merge_strategies ()
 		p
 	}'
 }
-__git_merge_strategylist=
-__git_merge_strategylist=$(__git_merge_strategies 2>/dev/null)
+
+__git_merge_strategies=
+# 'git merge -s help' (and thus detection of the merge strategy
+# list) fails, unfortunately, if run outside of any git working
+# tree.  __git_merge_strategies is set to the empty string in
+# that case, and the detection will be repeated the next time it
+# is needed.
+__git_compute_merge_strategies ()
+{
+	: ${__git_merge_strategies:=$(__git_list_merge_strategies)}
+}
 
 __git_complete_file ()
 {
@@ -474,27 +473,24 @@ __git_complete_remote_or_refspec ()
 
 __git_complete_strategy ()
 {
+	__git_compute_merge_strategies
 	case "${COMP_WORDS[COMP_CWORD-1]}" in
 	-s|--strategy)
-		__gitcomp "$(__git_merge_strategies)"
+		__gitcomp "$__git_merge_strategies"
 		return 0
 	esac
 	local cur="${COMP_WORDS[COMP_CWORD]}"
 	case "$cur" in
 	--strategy=*)
-		__gitcomp "$(__git_merge_strategies)" "" "${cur##--strategy=}"
+		__gitcomp "$__git_merge_strategies" "" "${cur##--strategy=}"
 		return 0
 		;;
 	esac
 	return 1
 }
 
-__git_all_commands ()
+__git_list_all_commands ()
 {
-	if [ -n "${__git_all_commandlist-}" ]; then
-		echo "$__git_all_commandlist"
-		return
-	fi
 	local i IFS=" "$'\n'
 	for i in $(git help -a|egrep '^  [a-zA-Z0-9]')
 	do
@@ -504,17 +500,18 @@ __git_all_commands ()
 		esac
 	done
 }
-__git_all_commandlist=
-__git_all_commandlist="$(__git_all_commands 2>/dev/null)"
 
-__git_porcelain_commands ()
+__git_all_commands=
+__git_compute_all_commands ()
+{
+	: ${__git_all_commands:=$(__git_list_all_commands)}
+}
+
+__git_list_porcelain_commands ()
 {
-	if [ -n "${__git_porcelain_commandlist-}" ]; then
-		echo "$__git_porcelain_commandlist"
-		return
-	fi
 	local i IFS=" "$'\n'
-	for i in "help" $(__git_all_commands)
+	__git_compute_all_commands
+	for i in "help" $__git_all_commands
 	do
 		case $i in
 		*--*)             : helper pattern;;
@@ -595,8 +592,13 @@ __git_porcelain_commands ()
 		esac
 	done
 }
-__git_porcelain_commandlist=
-__git_porcelain_commandlist="$(__git_porcelain_commands 2>/dev/null)"
+
+__git_porcelain_commands=
+__git_compute_porcelain_commands ()
+{
+	__git_compute_all_commands
+	: ${__git_porcelain_commands:=$(__git_list_porcelain_commands)}
+}
 
 __git_aliases ()
 {
@@ -1081,7 +1083,8 @@ _git_help ()
 		return
 		;;
 	esac
-	__gitcomp "$(__git_all_commands)
+	__git_compute_all_commands
+	__gitcomp "$__git_all_commands
 		attributes cli core-tutorial cvs-migration
 		diffcore gitk glossary hooks ignore modules
 		repository-layout tutorial tutorial-2
@@ -1427,7 +1430,8 @@ _git_config ()
 		return
 		;;
 	pull.twohead|pull.octopus)
-		__gitcomp "$(__git_merge_strategies)"
+		__git_compute_merge_strategies
+		__gitcomp "$__git_merge_strategies"
 		return
 		;;
 	color.branch|color.diff|color.interactive|\
@@ -1528,7 +1532,8 @@ _git_config ()
 	pager.*)
 		local pfx="${cur%.*}."
 		cur="${cur#*.}"
-		__gitcomp "$(__git_all_commands)" "$pfx" "$cur"
+		__git_compute_all_commands
+		__gitcomp "$__git_all_commands" "$pfx" "$cur"
 		return
 		;;
 	remote.*.*)
@@ -2125,7 +2130,8 @@ _git ()
 			--help
 			"
 			;;
-		*)     __gitcomp "$(__git_porcelain_commands) $(__git_aliases)" ;;
+		*)     __git_compute_porcelain_commands
+		       __gitcomp "$__git_porcelain_commands $(__git_aliases)" ;;
 		esac
 		return
 	fi