bash prompt: avoid command substitution when checking for untracked files

When enabled, the bash prompt can indicate the presence of untracked
files with a '%' sign.  __git_ps1() checks for untracked files by running the
'$(git ls-files --others --exclude-standard)' command substitution,
and displays the indicator when there is no output.

Avoid this command substitution by additionally passing
'--error-unmatch *', and checking the command's return value.

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

contrib/completion/git-prompt.sh

@@ -442,7 +442,7 @@ __git_ps1 ()
 
 		if [ -n "${GIT_PS1_SHOWUNTRACKEDFILES-}" ] &&
 		   [ "$(git config --bool bash.showUntrackedFiles)" != "false" ] &&
-		   [ -n "$(git ls-files --others --exclude-standard)" ]
+		   git ls-files --others --exclude-standard --error-unmatch -- '*' >/dev/null 2>/dev/null
 		then
 			u="%${ZSH_VERSION+%}"
 		fi