replace_object.c: rename to use dash in file name

replace_object.c: rename to use dash in file name

This is more consistent with the project style. The majority of
Git's source files use dashes in preference to underscores in their file
names.

Noticed while adding a header corresponding to this file.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>