Show different color and/or add a symbol to the branch column unless it shows the default

[strboul]

Show a different color and/or add a symbol to the branch column (like *) unless it shows the default branch name (mostly master, main, release etc.).

Unfortunately, this issue is a bit complex as there's no standard way of getting the default branch name right away.

---

Programmatically, it's possible to get the branch name in multiple ways:

1. If default branch is set in git config

git config --get init.defaultBranch

2. If the repo was downloaded with git clone

then origin/HEAD is set automatically.

git symbolic-ref refs/remotes/origin/HEAD | cut -d '/' -f4
master

git rev-parse --abbrev-ref origin/HEAD
origin/master

3. Get from the remote

git ls-remote --symref origin HEAD
ref: refs/heads/master  HEAD
28c8d9f868eb9af98618407170da6a63c43e7f62        HEAD

git ls-remote --symref origin HEAD
ref: refs/heads/main    HEAD
597b74a4a04c8741db1000901627745b65e5d69a        HEAD

The problem with this way is: It takes a while to get this from remote, like (~ 2000ms) so it's better
to create a cache file in the home directory (e.g. $HOME/.cache/git-substatus/cache.json)
to make the process faster.

Example cache file:

{
  "cache": [
    {
      "repository": "git-substatus",
      "defaultBranch": "master",
      "cacheTime": "1631264137"
    },
    {
      "repository": "test",
      "default-branch": "main",
      "cacheTime": "1631264137"
    }
  ]
}

The risk of caching is that the repository name isn't failproof as it may be replaced with another repository having the same name. That's why need to find a unique identifier for it?

The caching invalidation can happen every week (168h) and on every time when git-substatus is called with the --fetch option.

---

TODO:

• Detect default branch in a repository
• Display a visual cue if it's checked out to a non default branch
• Write unit tests (e.g. repos which default branch name isn't master)
• (Optional) Implement a caching mechanism to retrieve default branch information faster