git object store: make it possible to work with checked-out git branches

This makes the git object save logic also check out the changes in the
working tree and index if the branch we save to is checked out.  It used
to be that we would just update the object store (and the branch ref, of
course), but leave any checked-out state untouched.

Note that if the working directory is dirty (ie you have made changes by
hand and not committed them), the checkout will skip any dirty files and
report it as a warning to the user.  However, the save still succeeds
(since the _real_ save goes to the backing store).

NOTE NOTE NOTE! Both loading and saving very fundamentally work on the
git object store level, and if you are working with a checked-out branch
and make modifications to the working tree, saving will not touch those
dirty files (so that you can try to recover your edits manually in the
working tree), but it's worth pointing out that subsufrace loading state
will totally ignore the working tree.

So the only way to make subsurface *see* your changes is to commit them.
Having edited state checked out in the working tree will only confuse
you when subsurface first ignores it on reading, and then refuses to
touch the checked-out state on writing.

Put another way: working with a checked-out branch is now _possible_,
but you need to be aware of the limitations.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

qt-ui/mainwindow.cpp

@@ -909,6 +909,7 @@ int MainWindow::file_save_as(void)
 		return -1;
 	}
 
+	showError(get_error_string());
 	set_filename(filename.toUtf8().data(), true);
 	setTitle(MWTF_FILENAME);
 	mark_divelist_changed(false);
@@ -938,6 +939,7 @@ int MainWindow::file_save(void)
 		showError(get_error_string());
 		return -1;
 	}
+	showError(get_error_string());
 	mark_divelist_changed(false);
 	addRecentFile(QStringList() << QString(existing_filename));
 	return 0;