gitk: Add "--no-replace-objects" option

gitk: Add "--no-replace-objects" option

Replace refs are useful to change some git objects after they
have started to be shared between different repositories. One
might want to ignore them to see the original state, and
"--no-replace-objects" option can be used from the command
line to do so.

This option simply sets the GIT_NO_REPLACE_OBJECTS environment
variable, and that is enough to make gitk ignore replace refs.

The GIT_NO_REPLACE_OBJECTS is set to "1" instead of "" as it is
safer on some platforms, thanks to Johannes Sixt and Michael J
Gruber.

Tested-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>