Before, an absolute path would be saved in the git_target file if .git is a
symlink. That's not wrong, but seemed a bit weird. Let's consolidate the
behavior across .git file types.
Apparently, libgit2 doesn't deduce "core.bare" config from the directory name,
but gitoxide implements it correctly. So we shouldn't blindly canonicalize
the Git repository path. Fortunately, the saved git_target path isn't a fully-
canonicalized form (unless user explicitly sepcified "--git-repo ./.git"), so
we don't need a hack to remap git_target back to the symlink path.
is_colocated_git_workspace() is adjusted since the git_workdir is no longer
resolved from the fully-canonicalized repo path, at least in our code. Still we
have the ".git/.." fallback because test_init_git_colocated_symlink_gitlink()
would otherwise fail. I haven't figured out why, and the test might be actually
wrong compared to the git CLI behavior, but let's not change that for now.
Fixes#2668
A git repo created by the "repo" tool doesn't have core.base set, which means
the "bare"-ness relies on the directory name. Gitoxide appears to parse it
correctly, whereas libgit2 doesn't. That's why the symlinked .git repo is no
longer processed as a colocated repo.
#2668
Allowing `jj init --git` in an existing Git repo creates a second Git
store in `.jj/repo/store/git`, totally disconnected from the existing
Git store. This will only produce extremely confusing bugs for users,
since any operations they make in Git will *not* be reflected in the
jj repo.
If the existing git repo contains local and remote branches of the same name,
one of the remote branches is probably a tracking remote branch. Let's show
a hint how to set up tracking branches. The tracking state could be derived
from .git/config, but doing that automatically might cause another issue like
#1862, which could have been mitigated by git.auto-local-branch = false.
I've extracted the `builtin_log_root` template for users to customize the
default templates without fully overriding them, for example I would remove
the change_id/commit_id for myself - and we discussed in Discord that leaving
those makes sense for the user to be reminded/teached that the root commit has
a change id made from z's.
They are shown next to the change and commit id, since they are other names the
commit can be referred by.
The description is separated from the branches by a ` | ` when there are
branches, so that one can tell the branches from the description without color.
The result looks like this: 
Maybe we could load GitBackend without resolving .git symlink, but that would
introduce more subtle bugs. Instead, we calculate the expected Git workdir path
from the canonical ".git" path.
Fixes#2011
