dvc metrics diff --all: on same branch is empty

Bug Report

Description

Hello,
we found that dvc metrics diff --alloutputs nothing, if a_revand b_rev refer to the same git commit. No matter if they are exactly the same or different types of references (HEAD vs branch_name, etc.)

Reproduce

• setup any dvc stage with metrics.
• run dvc metrics diff --all $(git rev-parse --abbrev-ref HEAD) HEAD

Expected

output metrics-diff table with all values with diff = 0.0

Environment information

Output of dvc doctor:

-------------------------
Platform: Python 3.8.10 on Linux-6.5.0-28-generic-x86_64-with-glibc2.34
Subprojects:
        dvc_data = 2.22.6
        dvc_objects = 1.4.9
        dvc_render = 0.7.0
        dvc_task = 0.4.0
        scmrepo = 1.6.0
Supports:
        http (aiohttp = 3.9.3, aiohttp-retry = 2.8.3),
        https (aiohttp = 3.9.3, aiohttp-retry = 2.8.3),
        s3 (s3fs = 2024.3.1, boto3 = 1.34.51)
Config:
        Global: /home/mtunkowitsch/.config/dvc
        System: /etc/xdg/xdg-ubuntu/dvc
Cache types: hardlink, symlink
Cache directory: ext4 on /dev/nvme0n1p6
Caches: local
Remotes: s3
Workspace directory: ext4 on /dev/nvme0n1p6
Repo: dvc, git
Repo.site_cache_dir: /var/tmp/dvc/repo/6a15ed68432f0e8e4dba7e407082545a

Additional Information (if any):

Running the debugger, we noticed that metrics/diff.py:diff expects the results of metrics.show() to have the exact rev keys extracted. However metrics/show.py:show uses the brancher to extract the revs to use.
However the brancher groups revs with the same sha and joins them.

This means that when we call dvc metrics diff --all main main the brancher would group main and main and return main,main.
Hence the repo.metrics.show() outputs all metrics with the key main,main and the repo.metrics.diff() doesn't find results for main

Start with dvc/repo/metrics/diff.py and dvc/repo/metrics/show.py, then inspect the revision grouping in dvc/repo/brancher.py. Run the reported dvc metrics diff --all command with two references to the same commit and verify that the metrics-diff table includes all metrics with differences of 0.0.