fix(git): interpret bare HEAD through the shared resolver and stop overwriting .Upstream with the remote list

[e-kulikov]

Fixes #7799.

Prerequisites

• I have read and understood the contributing guide.
• The commit message follows the conventional commits guidelines.
• Tests for the changes have been added.
• Docs have been added/updated where relevant.

Description

getBareRepoInfo() hand-rolled a HEAD parse that only understood ref: refs/heads/… — a reftables
repository rendered .Ref as .invalid, and a detached bare HEAD rendered the full 40-character
object ID with .Detached false and .Hash/.ShortHash empty, even though the non-bare path already
handles all of this correctly. Separately, .Upstream was assigned the raw, newline-separated output
of git remote — with two remotes configured, that put a literal line break in the middle of the
prompt.

This PR routes bare HEAD interpretation through the same resolver the non-bare path already uses, and
replaces the .Upstream assignment with a real upstream/remote-selection lookup, resolving to the
branch's actual upstream (or empty, when none is configured) instead of the remote list. Most of the
directory-selection machinery this originally needed is no longer necessary, since the directory-role
fix below already makes the current checkout's git directory correct in every layout this PR touches.

Verified with new tests covering reftables, detached HEAD, single- and multi-remote bare repositories,
and the existing non-bare behaviour, confirmed unchanged.

Replaces #7803

#7803 was closed automatically when the next branch was deleted upstream — not because of anything
in the code. This is the same work, rebased onto main as a single commit, per the request to rebase
and squash. The earlier review discussion lives in #7803.

This is part of a stacked series

main (includes #7801, #7804)
 └─ fix/git-worktree-common-dir                 → #7828 (issue #7798)
     └─ fix/git-bare-head-upstream (this PR)    → issue #7799

This branch adds one commit on top of fix/git-worktree-common-dir (#7828) and depends on it
directly: the simplified HEAD resolution here assumes that PR's directory-role fix has landed.
Concretely, it relies on hasWorktree no longer repointing mainSCMDir at the working-tree root in
the --separate-git-dir branch — without that, updateHEADReference() reads HEAD from the wrong
directory and the bare-repository case in #7799 stays broken. Until #7828 merges, this PR's diff
includes its commit too; it will then shrink to this PR's own single commit.

[github-actions]

📦 Release binary size report

Compares this PR's release-equivalent build against the latest published release, per OS (amd64).

OS	Baseline	This PR	Delta
darwin	13.80 MB	13.80 MB	+4.0 KB (+0.03%)
linux	13.46 MB	13.46 MB	+4.0 KB (+0.03%)
windows	14.02 MB	13.99 MB	-38.3 KB (-0.27%)

🎉 Binary size shrank on at least one platform.