tree be02094f1da00acc4f894b17a5b0b385c6ab7271
parent 9ed4c4043a0f8b80a6629792a499c3b72bec08a8
author Youssef Elghareeb <ghareeb@google.com> 1624459188 +0200
committer Youssef Elghareeb <ghareeb@google.com> 1624461230 +0200

Diff against root commits: use ObjectId#zeroId instead of EMPTY_TREE_ID

We used to reserve the special EMPTY_TREE_ID
4b825dc642cb6eb9a060e54bf8d69288fbee4904 for the
FileDiffCacheKey#oldCommit and the GitFileDiffCacheKey#oldTree. We also
use the same thing for the ModifiedFilesCache and GitModifiedFilesCache.
Callers pass this value to request the diff for a root commit. The cache
loader used to pass this value further to DiffFormatter#scan (in JGit)
to perform the scan.

Some repositories may not have the EMPTY_TREE_ID as a valid git object.
This resulted in a MissingObjectException in DiffFormatter#scan.

Here we change the following:
1) Use ObjectId#zeroId instead of EMPTY_TREE_ID in the cache keys to
signal requesting a diff against an added tree (i.e. root commit).
2) Change the handling in the loader by passing a null value to the old
tree parameter in DiffFormatter#scan. The method documents accepting
null values to perform this kind of diff. Check [1].

[1] https://git.eclipse.org/r/plugins/gitiles/jgit/jgit/+/f598e69529e0a1864e8224265ed82326f2a296f5/org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffFormatter.java#426

Change-Id: I004dcf991f600052054aa167f24a925f1994eab3