MinGW: fix diff --no-index /dev/null ...
When launching "diff --no-index" with a parameter "/dev/null", the MSys
bash converts the "/dev/null" to a "nul", which usually makes sense. But
diff --no-index got confused and tried to access a _file_ called "nul".
While at it, the comment in t4012, expressed as ":# <text>" was turned
into ": <text>" so that MSys' path name mangling does not kick in.
With this patch, t4012 passes in msysGit.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>