Please create a better display for git mv (BB-11223)

Right now, the results of a git mv in a commit are shown in bitbucket as if the previous file was removed and a new file was added. However, it's important that the commit display show that a git mv operation happened, because git mv preserves line-by-line annotations whereas a cp / git rm / git add does not. Further, cosmetically, this clutters up the diff by potentially hundreds or thousands of lines that could be boiled down into a single line.