git-gui: use git rev-parse for worktree discovery

git-gui: use git rev-parse for worktree discovery

git gui uses a combination of tcl code and git invocations to determine
the worktree and the location with respect to the worktree root
(_prefix). But, git rev-parse provides all of this information directly,
and assures full error and configuration checking are done by git
itself. The entirety of discovery in normal configurations involves

	git rev-parse --show-toplevel (gets worktree root)
	git rev-parse --show-prefix (shows location wrt the root)

An error thrown on either of these lines means the worktree discovered
by git is unusable, or git did not discover a worktree because the
current directory is inside the repository. If the user has defined
GIT_DIR or GIT_WORK_TREE, this is a user configuration error and git-gui
should stop.

Otherwise, the blame or browser subcommands can be used without a
worktree.

A separate error might occur when changing to the root of the discovered
worktree. The cause would be file system related and completely outside
of git's control, so trap that independently.

Discovery of the repository and the worktree must be guarded to trap
errors: the intent is that any configuration problems are caught during
discovery, and later processing need not include error trapping and
recovery. So, move all worktree discovery code to be immediately after
repository discovery.

This does move configuration loading to occur after worktree discovery
rather than before. None of the code executed in worktree discovery has
any option controlled by a git-gui configuration variable, so no impact
is expected. git itself will always read the repository configuration,
including worktree specific configuration data if that exists, so this
is unaffected by when git-gui loads its own config data. Also, we cannot
be sure the worktree dependent configuration can be loaded before
full discovery is complete.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>