bash prompt: don't use '+=' operator in show upstream code path

The '+=' operator is not supported by old Bash versions (3.0) we still
care about.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

@@ -110,7 +110,7 @@ __git_ps1_show_upstream ()
 			;;
 		svn-remote.*.url)
 			svn_remote[$((${#svn_remote[@]} + 1))]="$value"
-			svn_url_pattern+="\\|$value"
+			svn_url_pattern="$svn_url_pattern\\|$value"
 			upstream=svn+git # default upstream is SVN if available, else git
 			;;
 		esac