DiffParser.get_orig_commit_id() was an old hack originally intended

for solving a complex issue with parent diffs for Mercurial. To help

understand it, we need to dive into the history of how parent diffs,

source revisions, and file lookups worked.
Originally, when parsing a diff + parent diff, we'd set the source

revision for a FileDiff to that of the source of the file for the

parent diff, enabling us to look up the right soure file. We'd then

apply the parent diff and then the diff on top of it. If a file didn't

exist in a parent diff, we could just use the source revision in the

main diff, since we could assume it would be present in the repository.
The problem at that point was that, if the SCM used a commit revision to

look up files, and the file wasn't present in the parent diff, we

wouldn't have a suitable revision to use. To address this,

DiffParser.get_orig_commit_id() was introduced. This would allow the

diff parser to capture the parent revision, and then we could use that

captured revision from the parent diff as the source revision for files.
More recently, we stopped assigning a parent diff's source revision to

the FileDiff, and instead preserved the main diff's source revisions.

We'd then store parent diff revisions and filenames in extra_data.

This was part of an effort to fix some file lookup issues and file

matching involving parent diffs. However, we still needed

get_orig_commit_id() to get those parent commit IDs, and we were still

storing them as the FileDiff's source revision.
That brings us to this change.
Now that we have the new parsed diff objects (ParsedDiff and

ParsedDiffChange), we have a new avenue for storing useful information

during parsing. This change adds commit_id and parent_commit_id to

ParsedDiffChange, which can be optionally set by the parser. It also

adds a capability flag, uses_commit_ids_as_revisions, to ParsedDiff,

which can also be set.
If this flag is set, and commit ID information is recorded, then it will

be used as the parent source revision when building the FileDiffs.

This works just like get_orig_commit_id(), except we're no longer

storing the value in extra_data like other parent source revisions.
get_orig_commit_id() is now considered deprecated, scheduled to be

removed in Review Board 5.0. DiffParser will check for a value after

parsing and, if found, will set it and the

uses_commit_ids_as_revisions flag, emitting a deprecation warning in

the process.
This is a big cleanup that addresses one of the worst remnants of our

older diff parsing code, and opens the door to allowing a DiffX parser

to provide the same parent revision behavior.

Unit tests passed.
Tested posting a Mercurial diff with a parent diff. Verified that the

result was correct and contained the right revision metadata. This test

also instrumented the compatibility for get_orig_commit_id(), since

the Mercurial implementation has not been updated.