Adding "ahead/behind" info to `status`

[mathomp4]

This is a new functionality requested by @sdrabenh.

He was wondering if there is a way for mepo to display if your repos are behind (or ahead) of its remote. So a user can do mepo fetch to fetch all the repos. And then we could have:

❯ mepo status
Checking status...
MAPL       | (b) release/MAPL-v3
ESMA_env   | (t) v4.35.0 (DH)
ESMA_cmake | (b) testing-behind [stashes: 1] [ahead 1, behind 1]
ecbuild    | (t) geos/v1.4.0 (DH)

I think this is doable via:

git for-each-ref --format="%(upstream:track)" refs/heads/$(git symbolic-ref --short HEAD)

which on the above ESMA_cmake repo actually outputs:

❯ git for-each-ref --format="%(upstream:track)" refs/heads/$(git symbolic-ref --short HEAD)
[ahead 1, behind 1]

I'm not sure if for "niceness" we should have it be [stashes 1] as well, but...eh. The message is there.

Also, maybe the ahead/behind can be in orange? Or some nice color to stand out?

[pchakraborty]

I did this a little differently - parsed the headers of the output of git status --porcelain=v2 --branch --show-stash.