git: add --no-replace-objects option to disable replacing

Commit dae556b (environment: add global variable to disable replacement)
adds a variable to enable/disable replacement, and it is enabled by
default for most commands.

So there is no way to disable it for some commands, which is annoying
when we want to get information about a commit that has been replaced.

For example:

$ git cat-file -p N

would output information about the replacement commit if commit N is
replaced.

With the "--no-replace-objects" option that this patch adds it is
possible to get information about the original commit using:

$ git --no-replace-objects cat-file -p N

While at it, let's add some documentation about this new option in the
"git replace" man page too.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6050-replace.sh

@@ -70,6 +70,13 @@ test_expect_success 'replace the author' '
      git show $HASH2 | grep "O Thor"
 '
 
+test_expect_success 'test --no-replace-objects option' '
+     git cat-file commit $HASH2 | grep "author O Thor" &&
+     git --no-replace-objects cat-file commit $HASH2 | grep "author A U Thor" &&
+     git show $HASH2 | grep "O Thor" &&
+     git --no-replace-objects show $HASH2 | grep "A U Thor"
+'
+
 cat >tag.sig <<EOF
 object $HASH2
 type commit