interpret_branch_name: allow callers to restrict expansions

The interpret_branch_name() function converts names like
@{-1} and @{upstream} into branch names. The expanded ref
names are not fully qualified, and may be outside of the
refs/heads/ namespace (e.g., "@" expands to "HEAD", and
"@{upstream}" is likely to be in "refs/remotes/").

This is OK for callers like dwim_ref() which are primarily
interested in resolving the resulting name, no matter where
it is. But callers like "git branch" treat the result as a
branch name in refs/heads/.  When we expand to a ref outside
that namespace, the results are very confusing (e.g., "git
branch @" tries to create refs/heads/HEAD, which is
nonsense).

Callers can't know from the returned string how the
expansion happened (e.g., did the user really ask for a
branch named "HEAD", or did we do a bogus expansion?). One
fix would be to return some out-parameters describing the
types of expansion that occurred. This has the benefit that
the caller can generate precise error messages ("I
understood @{upstream} to mean origin/master, but that is a
remote tracking branch, so you cannot create it as a local
name").

However, out-parameters make the function interface somewhat
cumbersome. Instead, let's do the opposite: let the caller
tell us which elements to expand. That's easier to pass in,
and none of the callers give more precise error messages
than "@{upstream} isn't a valid branch name" anyway (which
should be sufficient).

The strbuf_branchname() function needs a similar parameter,
as most of the callers access interpret_branch_name()
through it.

We can break the callers down into two groups:

  1. Callers that are happy with any kind of ref in the
     result. We pass "0" here, so they continue to work
     without restrictions. This includes merge_name(),
     the reflog handling in add_pending_object_with_path(),
     and substitute_branch_name(). This last is what powers
     dwim_ref().

  2. Callers that have funny corner cases (mostly in
     git-branch and git-checkout). These need to make use of
     the new parameter, but I've left them as "0" in this
     patch, and will address them individually in follow-on
     patches.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -452,7 +452,7 @@ static void setup_branch_path(struct branch_info *branch)
 {
 	struct strbuf buf = STRBUF_INIT;
 
-	strbuf_branchname(&buf, branch->name);
+	strbuf_branchname(&buf, branch->name, 0);
 	if (strcmp(buf.buf, branch->name))
 		branch->name = xstrdup(buf.buf);
 	strbuf_splice(&buf, 0, 0, "refs/heads/", 11);