remote.c: introduce branch_get_upstream helper

All of the information needed to find the @{upstream} of a
branch is included in the branch struct, but callers have to
navigate a series of possible-NULL values to get there.
Let's wrap that logic up in an easy-to-read helper.

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

builtin/branch.c

@@ -123,14 +123,12 @@ static int branch_merged(int kind, const char *name,
 
 	if (kind == REF_LOCAL_BRANCH) {
 		struct branch *branch = branch_get(name);
+		const char *upstream = branch_get_upstream(branch);
 		unsigned char sha1[20];
 
-		if (branch &&
+		if (upstream &&
-		    branch->merge &&
-		    branch->merge[0] &&
-		    branch->merge[0]->dst &&
 		    (reference_name = reference_name_to_free =
-		     resolve_refdup(branch->merge[0]->dst, RESOLVE_REF_READING,
+		     resolve_refdup(upstream, RESOLVE_REF_READING,
 				    sha1, NULL)) != NULL)
 			reference_rev = lookup_commit_reference(sha1);
 	}

builtin/for-each-ref.c

@@ -664,10 +664,9 @@ static void populate_value(struct refinfo *ref)
 				continue;
 			branch = branch_get(ref->refname + 11);
 
-			if (!branch || !branch->merge || !branch->merge[0] ||
+			refname = branch_get_upstream(branch);
-			    !branch->merge[0]->dst)
+			if (!refname)
 				continue;
-			refname = branch->merge[0]->dst;
 		} else if (starts_with(name, "color:")) {
 			char color[COLOR_MAXLEN] = "";
 

builtin/log.c

@@ -1632,16 +1632,13 @@ int cmd_cherry(int argc, const char **argv, const char *prefix)
 		break;
 	default:
 		current_branch = branch_get(NULL);
-		if (!current_branch || !current_branch->merge
+		upstream = branch_get_upstream(current_branch);
-					|| !current_branch->merge[0]
+		if (!upstream) {
-					|| !current_branch->merge[0]->dst) {
 			fprintf(stderr, _("Could not find a tracked"
 					" remote branch, please"
 					" specify <upstream> manually.\n"));
 			usage_with_options(cherry_usage, options);
 		}
-
-		upstream = current_branch->merge[0]->dst;
 	}
 
 	init_revisions(&revs, prefix);

remote.c

@@ -1705,6 +1705,13 @@ int branch_merge_matches(struct branch *branch,
 	return refname_match(branch->merge[i]->src, refname);
 }
 
+const char *branch_get_upstream(struct branch *branch)
+{
+	if (!branch || !branch->merge || !branch->merge[0])
+		return NULL;
+	return branch->merge[0]->dst;
+}
+
 static int ignore_symref_update(const char *refname)
 {
 	unsigned char sha1[20];
@@ -1914,12 +1921,11 @@ int stat_tracking_info(struct branch *branch, int *num_ours, int *num_theirs)
 	int rev_argc;
 
 	/* Cannot stat unless we are marked to build on top of somebody else. */
-	if (!branch ||
+	base = branch_get_upstream(branch);
-	    !branch->merge || !branch->merge[0] || !branch->merge[0]->dst)
+	if (!base)
 		return 0;
 
 	/* Cannot stat if what we used to build on no longer exists */
-	base = branch->merge[0]->dst;
 	if (read_ref(base, sha1))
 		return -1;
 	theirs = lookup_commit_reference(sha1);

remote.h

@@ -218,6 +218,13 @@ const char *pushremote_for_branch(struct branch *branch, int *explicit);
 int branch_has_merge_config(struct branch *branch);
 int branch_merge_matches(struct branch *, int n, const char *);
 
+/**
+ * Return the fully-qualified refname of the tracking branch for `branch`.
+ * I.e., what "branch@{upstream}" would give you. Returns NULL if no
+ * upstream is defined.
+ */
+const char *branch_get_upstream(struct branch *branch);
+
 /* Flags to match_refs. */
 enum match_refs_flags {
 	MATCH_REFS_NONE		= 0,