Fix sed expression for OS X and Solaris 2.6.

Matches previous fixes in other tests.

tests/test-diff-ignore-whitespace

@@ -1,84 +1,85 b''
 #!/bin/sh
 
 # GNU diff is the reference for all of these results.
 
 hgdiff() {
-    hg diff "$@" | sed "s/\(\(---\|+++\) [a-zA-Z0-9_/.-]*\).*/\1/"
+    hg diff "$@" | sed -e "s/\(+++ [a-zA-Z0-9_/.-]*\).*/\1/" \
+                  -e "s/\(--- [a-zA-Z0-9_/.-]*\).*/\1/"
 }
 
 test_added_blank_lines() {
     printf '\nhello world\n\ngoodbye world\n\n' >foo
 
     echo '>>> two diffs showing three added lines <<<'
     hgdiff
     hgdiff -b
 
     echo '>>> no diffs <<<'
     hgdiff -B
     hgdiff -Bb
 }
 
 test_added_horizontal_space_first_on_a_line() {
     printf '\t hello world\ngoodbye world\n' >foo
 
     echo '>>> four diffs showing added space first on the first line <<<'
     hgdiff
     hgdiff -b
     hgdiff -B
     hgdiff -Bb
 }
 
 test_added_horizontal_space_last_on_a_line() {
     printf 'hello world\t \ngoodbye world\n' >foo
 
     echo '>>> two diffs showing space appended to the first line <<<'
     hgdiff
     hgdiff -B
 
     echo '>>> no diffs <<<'
     hgdiff -b
     hgdiff -Bb
 }
 
 test_added_horizontal_space_in_the_middle_of_a_word() {
     printf 'hello world\ngood bye world\n' >foo
 
     echo '>>> four diffs showing space inserted into "goodbye" <<<'
     hgdiff
     hgdiff -B
     hgdiff -b
     hgdiff -Bb
 }
 
 test_increased_horizontal_whitespace_amount() {
     printf 'hello world\ngoodbye\t\t  \tworld\n' >foo
 
     echo '>>> two diffs showing changed whitespace amount in the last line <<<'
     hgdiff
     hgdiff -B
 
     echo '>>> no diffs <<<'
     hgdiff -b
     hgdiff -Bb
 }
 
 test_added_blank_line_with_horizontal_whitespace() {
     printf 'hello world\n \t\ngoodbye world\n' >foo
 
     echo '>>> four diffs showing added blank line w/horizontal space <<<'
     hgdiff
     hgdiff -B
     hgdiff -b
     hgdiff -Bb
 }
 
 hg init
 printf 'hello world\ngoodbye world\n' >foo
 hg ci -Amfoo -ufoo -d '0 0'
 
 test_added_blank_lines
 test_added_horizontal_space_first_on_a_line
 test_added_horizontal_space_last_on_a_line
 test_added_horizontal_space_in_the_middle_of_a_word
 test_increased_horizontal_whitespace_amount
 test_added_blank_line_with_horizontal_whitespace