Failure of --diff-cmd silently ignored

[Daniel Shahaf]

[[[
% $svn di --diff-cmd =false
Index: subversion/include/svn_cache_config.h
===================================================================
Index: subversion/libsvn_subr/deprecated.c
===================================================================
Index: subversion/libsvn_wc/wc_db.c
===================================================================
% echo $?
0
]]]

(where '=false' expands to what `which false` does)