bash prompt: avoid command substitution when finalizing gitstring

Before setting $PS1, __git_ps1() uses a command substitution to
redirect the output from a printf into a variable.  Spare the overhead
of fork()ing a subshell by using 'printf -v <var>' to directly assign
the output to that variable.

zsh's printf doesn't support the '-v <var>' option, so stick with the
command substitution when under zsh.

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

contrib/completion/git-prompt.sh

@@ -461,7 +461,11 @@ __git_ps1 ()
 		else
 			gitstring="$c${b##refs/heads/}${f:+$z$f}$r$p"
 		fi
-		gitstring=$(printf -- "$printf_format" "$gitstring")
+		if [[ -n ${ZSH_VERSION-} ]]; then
+			gitstring=$(printf -- "$printf_format" "$gitstring")
+		else
+			printf -v gitstring -- "$printf_format" "$gitstring"
+		fi
 		PS1="$ps1pc_start$gitstring$ps1pc_end"
 	else
 		# NO color option unless in PROMPT_COMMAND mode