gitweb test: fix highlight test hang on Linux Mint

Linux Mint has an implementation of the highlight command (unrelated to the one from http://www.andre-simon.de) that works as a simple filter. The script uses 'sed' to add terminal colour escape codes around text matching a regular expression. When t9500-*.sh attempts to run "highlight --version", the script simply hangs waiting for input. (See https://bugs.launchpad.net/linuxmint/+bug/815005). The tool required by gitweb can be installed from the 'highlight' package. Unfortunately, given the default $PATH, this leads to the tool having lower precedence than the script. In order to avoid hanging the test, add '</dev/null' to the command line of the highlight invocation. Also, since the 'highlight' tool requred by gitweb produces '--version' output (and the script does not), saving the command output allows a simple check for the wrong 'highlight'. Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>