lib/repo-pull: Rename a variable for clarity

lib/repo-pull: Rename a variable for clarity

In libostree, the phrase "commit metadata" has two meanings-- one is the
first dictionary in a commit GVariant that stores metadata such as ref
bindings, and the other is the commit metadata in the summary file,
which stores the commit size, checksum, and timestamp. In
find_remotes_process_refs(), the entire commit GVariant was being
referred to as commit metadata, so this commit changes the variable
name and a comment to make things more consistent.

Closes: #1528
Approved by: cgwalters