Add a parameter to retrieve full context diffs via Compare API
### Problem to solve
When diffing branches or commits via the Compare API, the unified diffs are return with partial context. To show a full context view of the changes, it requires retrieving the full contents of each file via the Files API. This is quite costly when the number of changed files is large.
### Intended users
Tool developers.
### Proposal
#### Option 1: Context parameter
Accept an optional `context` parameter for `GET /projects/:id/repository/compare` that is passed to `git diff`
#### Option 2: Snapshot parameter
Accept an optional `snapshot` parameter for `GET /projects/:id/repository/compare` which is `false` by default. If `true` is provided, a `$.diffs[*].snapshot` field is provided with full content (preferred left side of the comparison).
### What does success look like, and how can we measure that?
Possible to create/retrieve full context diffs from a single Compare API without additional File API calls.
issue