In "jj absorb", we'll need to calculate annotation from the parent tree. It's
usually identical to the tree of the parent commit, but this is not true for a
merge commit. Since I'm not sure how we'll process conflict trees in general,
this patch adds a minimal API to specify a single file content, not a
MergedTree.
The primary use case is to exclude immutable commits when calculating line
ranges to absorb. For example, "jj absorb" will build annotation of @ revision
with domain = mutable().
We might want to calculate (commit_id, range) pairs of consecutive lines in
order to "absorb" changes, for example.
This should also be cheaper since Vec<u8> doesn't have to be allocated per line.
