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>