contrib/git-prompt.sh: handle missing 'printf -v' more gracefully

Old Bash (3.0) which is distributed with RHEL 4.X and other ancient
platforms that are still in wide use, do not have a printf that
supports -v.  Neither does Zsh (which is already handled in the code).

As suggested by Junio, let's test whether printf supports the -v
option and store the result.  Then later, we can use it to
determine whether 'printf -v' can be used, or whether printf
must be called in a subshell.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

diff --git a/contrib/completion/git-prompt.sh b/contrib/completion/git-prompt.sh
index a81ef5a..d6c61b2 100644
--- a/contrib/completion/git-prompt.sh
+++ b/contrib/completion/git-prompt.sh
@@ -84,6 +84,10 @@
 # the colored output of "git status -sb" and are available only when
 # using __git_ps1 for PROMPT_COMMAND or precmd.
 
+# check whether printf supports -v
+__git_printf_supports_v=
+printf -v __git_printf_supports_v -- '%s' yes >/dev/null 2>&1
+
 # stores the divergence from upstream in $p
 # used by GIT_PS1_SHOWUPSTREAM
 __git_ps1_show_upstream ()
@@ -433,7 +437,7 @@
 	local gitstring="$c${b##refs/heads/}${f:+$z$f}$r$p"
 
 	if [ $pcmode = yes ]; then
-		if [[ -n ${ZSH_VERSION-} ]]; then
+		if [ "${__git_printf_supports_v-}" != yes ]; then
 			gitstring=$(printf -- "$printf_format" "$gitstring")
 		else
 			printf -v gitstring -- "$printf_format" "$gitstring"