When running diff commands, a pathspec containing decomposed
unicode code points is not converted to precomposed unicode form
under Mac OS X, but we normalize the paths in the index and the
history to precomposed form on that platform.  As a result, the
pathspec would not match and no diff is shown.

Unlike many builtin commands, the "diff" family of commands do
not use parse_options(), which is how other builtin commands
indirectly call precompose_argv() to normalize argv[] into
precomposed form on Mac OSX.  Teach these commands to call
precompose_argv() themselves.

Note that precomopose_argv() normalizes not just paths but all
command line arguments, so things like "git diff -G $string"
when $string has the decomposed form would first be normalized
into the precomposed form and would stop hitting the same string
in the decomposed form in the diff output with this change.

It is not a problem per-se, as "log" family of commands already use
parse_options() and call precompose_argv()--we can think of this
change as making the "diff" family of commands behave in a similar
way as the commands in the "log" family.

Signed-off-by: Alexander Rinass <alex@fournova.com>
Helped-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>