test-dispatch: Make test of removed working directory work on AIX (issue2315)

AIX sh won't delete its own working directory. Removing it from another process
works.

Also hide the actual OS error message - operating systems returns different
errors when getcwd fails.

tests/test-dispatch

@@ -22,8 +22,8 b' EOF'
 hg cat a
 
 echo '% working directory removed'
-rm -rf $dir/a
-hg --version
+sh -c "cd $dir && rm -rf a"
+hg --version 2>&1 | sed -e 's,\(abort:.*:\).*$,\1 ...,g'
 
 echo '% no repo'
 cd $dir

tests/test-dispatch.out

@@ -34,6 +34,6 b' use "hg -v help cat" to show global opti'
 a
 a: No such file in rev 000000000000
 % working directory removed
-abort: error getting current working directory: No such file or directory
+abort: error getting current working directory: ...
 % no repo
 abort: There is no Mercurial repository here (.hg not found)!