[PATCH v2 10/13] bash prompt: combine 'git rev-parse' executions

["SZEDER Gábor" <szeder@ira.uka.de>]

From: SZEDER Gábor <szeder@ira.uka.de>

>From the four '$(git rev-parse --<opt>)' command substitutions
__git_ps1() executes three in its 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 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.

The whole series speeds up the bash prompt on Windows/MSysGit
considerably.  Here are some timing results in two scenarios, repeated
10 times:

At the top of the work tree, before:

    $ time for i in {0..9} ; do prompt="$(__git_ps1)" ; done

    real    0m1.716s
    user    0m0.301s
    sys     0m0.772s

  After:

    real    0m0.686s
    user    0m0.075s
    sys     0m0.287s

In a subdirectory, during rebase, stash status indicator enabled,
before:

    real    0m3.557s
    user    0m0.495s
    sys     0m1.767s

  After:

    real    0m0.702s
    user    0m0.045s
    sys     0m0.409s

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
---
 contrib/completion/git-prompt.sh | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/contrib/completion/git-prompt.sh b/contrib/completion/git-prompt.sh
index 10ec6902..b73cebf7 100644
--- a/contrib/completion/git-prompt.sh
+++ b/contrib/completion/git-prompt.sh
@@ -311,8 +311,9 @@ __git_ps1 ()
 		;;
 	esac
 
-	local g=
-	if ! g="$(git rev-parse --git-dir 2>/dev/null)"; then
+	local repo_info=
+	if ! repo_info="$(git rev-parse --git-dir --is-inside-git-dir \
+		--is-bare-repository --is-inside-work-tree 2>/dev/null)"; then
 		if [ $pcmode = yes ]; then
 			#In PC mode PS1 always needs to be set
 			PS1="$ps1pc_start$ps1pc_end"
@@ -320,6 +321,13 @@ __git_ps1 ()
 		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=""
@@ -396,13 +404,13 @@ __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
-- 
1.8.3.1.487.g8f4672d