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>

t/t9500-gitweb-standalone-no-errors.sh

@@ -654,9 +654,11 @@ cat >>gitweb_config.perl <<\EOF
 $feature{'highlight'}{'override'} = 1;
 EOF
 
-highlight --version >/dev/null 2>&1
+highlight_version=$(highlight --version </dev/null 2>/dev/null)
 if [ $? -eq 127 ]; then
-	say "Skipping syntax highlighting test, because 'highlight' was not found"
+	say "Skipping syntax highlighting tests: 'highlight' not found"
+elif test -z "$highlight_version"; then
+	say "Skipping syntax highlighting tests: incorrect 'highlight' found"
 else
 	test_set_prereq HIGHLIGHT
 fi