Refactor fast-import branch creation from existing commit

To resolve a corner case uncovered by Simon Hausmann I need to
reuse the logic for the SHA-1 expression version of the 'from '
command within the mark version of the 'from ' command.  This change
doesn't alter any functionality, but is merely breaking the common
code out to a function that I can reuse.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

fast-import.c

@@ -1657,6 +1657,33 @@ static void file_change_deleteall(struct branch *b)
 	load_tree(&b->branch_tree);
 }
 
+static void cmd_from_commit(struct branch *b, char *buf, unsigned long size)
+{
+	if (!buf || size < 46)
+		die("Not a valid commit: %s", sha1_to_hex(b->sha1));
+	if (memcmp("tree ", buf, 5)
+		|| get_sha1_hex(buf + 5, b->branch_tree.versions[1].sha1))
+		die("The commit %s is corrupt", sha1_to_hex(b->sha1));
+	hashcpy(b->branch_tree.versions[0].sha1,
+		b->branch_tree.versions[1].sha1);
+}
+
+static void cmd_from_existing(struct branch *b)
+{
+	if (is_null_sha1(b->sha1)) {
+		hashclr(b->branch_tree.versions[0].sha1);
+		hashclr(b->branch_tree.versions[1].sha1);
+	} else {
+		unsigned long size;
+		char *buf;
+
+		buf = read_object_with_reference(b->sha1,
+			commit_type, &size, b->sha1);
+		cmd_from_commit(b, buf, size);
+		free(buf);
+	}
+}
+
 static void cmd_from(struct branch *b)
 {
 	const char *from;
@@ -1688,34 +1715,11 @@ static void cmd_from(struct branch *b)
 			die("Mark :%" PRIuMAX " not a commit", idnum);
 		hashcpy(b->sha1, oe->sha1);
 		buf = gfi_unpack_entry(oe, &size);
-		if (!buf || size < 46)
-			die("Not a valid commit: %s", from);
-		if (memcmp("tree ", buf, 5)
-			|| get_sha1_hex(buf + 5, b->branch_tree.versions[1].sha1))
-			die("The commit %s is corrupt", sha1_to_hex(b->sha1));
+		cmd_from_commit(b, buf, size);
 		free(buf);
-		hashcpy(b->branch_tree.versions[0].sha1,
-			b->branch_tree.versions[1].sha1);
-	} else if (!get_sha1(from, b->sha1)) {
-		if (is_null_sha1(b->sha1)) {
-			hashclr(b->branch_tree.versions[0].sha1);
-			hashclr(b->branch_tree.versions[1].sha1);
-		} else {
-			unsigned long size;
-			char *buf;
-
-			buf = read_object_with_reference(b->sha1,
-				commit_type, &size, b->sha1);
-			if (!buf || size < 46)
-				die("Not a valid commit: %s", from);
-			if (memcmp("tree ", buf, 5)
-				|| get_sha1_hex(buf + 5, b->branch_tree.versions[1].sha1))
-				die("The commit %s is corrupt", sha1_to_hex(b->sha1));
-			free(buf);
-			hashcpy(b->branch_tree.versions[0].sha1,
-				b->branch_tree.versions[1].sha1);
-		}
-	} else
+	} else if (!get_sha1(from, b->sha1))
+		cmd_from_existing(b);
+	else
 		die("Invalid ref name or SHA1 expression: %s", from);
 
 	read_next_command();