Show changes to execute bit status in diffs

Currently, the Bitbucket PR few does not show changes in the execute bit setting on a file. This means that the following kinds of errors are hard to detect as part of a code review:

• scripts and Git LFS binaries being added without having their execute bits set correctly
• scripts and Git LFS binaries losing their set execute bit when moved or renamed
• other files incorrectly having their execute bit set

Git traditionally shows changes to the execute bit status by reporting a change in file permissions from 0644 to 0755 (or vice-versa), since checked out files are set as 0644 by default in the client, and only the execute bit is stored in the repo to request 0755 instead.

It would be perfectly fine if the Bitbucket UI support for this feature followed the same file permissions idiom that git itself uses (which was the request in BCLOUD-11961, but that was unfortunately misinterpreted as a request to store the full file permissions, and hence closed without action).