Construct the URL Manually: https://dev.azure.com/{organization}/{project}/_git/{repository}/branchCompare?baseVersion=GC{commitA}&targetVersion=GC{commitB}&_a=files

you specify the source base and target version in the url query string.

also you can Mix and match versions

GB for a branch name GT for a tag

once you entered the url and load the page you will see a full diff view that lists all file changes between the two commits.