bash prompt: combine 'git rev-parse' executions in the main code path

There are a couple of '$(git rev-parse --<opt>)' command substitutions
in __git_ps1() and three of them are executed in the main code path:

 - the first to get the path to the .git directory ('--git-dir'),
 - the second to check whether we're inside the .git directory
   ('--is-inside-git-dir'),
 - and the last, depending on the results of the second, either
   * to check whether it's a bare repo ('--is-bare-repository'), or
   * to check whether inside a work tree ('--is-inside-work-tree').

Naturally, this imposes the overhead of fork()ing three subshells and
fork()+exec()ing three git commands.

Combine these four 'git rev-parse' queries into a single one and use
bash parameter expansions to parse the combined output, i.e. to
separate the path to the .git directory from the true/false of
'--is-inside-git-dir', etc.  This way we can eliminate two of the
three subshells and git commands.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>

contrib/completion/git-prompt.sh

@@ -311,15 +311,23 @@ __git_ps1 ()
 		;;
 	esac
 
-	local g="$(git rev-parse --git-dir 2>/dev/null)"
-	if [ -z "$g" ]; then
+	local repo_info="$(git rev-parse --git-dir --is-inside-git-dir \
+		--is-bare-repository --is-inside-work-tree 2>/dev/null)"
+	if [ -z "$repo_info" ]; then
 		if [ $pcmode = yes ]; then
 			#In PC mode PS1 always needs to be set
 			PS1="$ps1pc_start$ps1pc_end"
 		fi
 		return
 	fi
 
+	local inside_worktree="${repo_info##*$'\n'}"
+	repo_info="${repo_info%$'\n'*}"
+	local bare_repo="${repo_info##*$'\n'}"
+	repo_info="${repo_info%$'\n'*}"
+	local inside_gitdir="${repo_info##*$'\n'}"
+	local g="${repo_info%$'\n'*}"
+
 	local r=""
 	local b=""
 	local step=""
@@ -402,13 +410,13 @@ __git_ps1 ()
 	local c=""
 	local p=""
 
-	if [ "true" = "$(git rev-parse --is-inside-git-dir 2>/dev/null)" ]; then
-		if [ "true" = "$(git rev-parse --is-bare-repository 2>/dev/null)" ]; then
+	if [ "true" = "$inside_gitdir" ]; then
+		if [ "true" = "$bare_repo" ]; then
 			c="BARE:"
 		else
 			b="GIT_DIR!"
 		fi
-	elif [ "true" = "$(git rev-parse --is-inside-work-tree 2>/dev/null)" ]; then
+	elif [ "true" = "$inside_worktree" ]; then
 		if [ -n "${GIT_PS1_SHOWDIRTYSTATE-}" ] &&
 		   [ "$(git config --bool bash.showDirtyState)" != "false" ]
 		then