cmd_merge(): Parse options before checking MERGE_HEAD

Reorder the initial part of builtin/merge.c:cmd_merge() so that command-line
options are parsed _before_ we load the index and check for MERGE_HEAD
(and exits if it exists). This does not change the behaviour of 'git merge',
but is needed in preparation for the implementation of 'git merge --abort'
(which requires MERGE_HEAD to be present).

This patch has been improved by the following contributions:
- Junio C Hamano: fixup minor style issues

Thanks-to: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

diff --git a/builtin/merge.c b/builtin/merge.c
index 37ce4f5..478a492 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -895,22 +895,6 @@
 	const char *best_strategy = NULL, *wt_strategy = NULL;
 	struct commit_list **remotes = &remoteheads;
 
-	if (read_cache_unmerged()) {
-		die_resolve_conflict("merge");
-	}
-	if (file_exists(git_path("MERGE_HEAD"))) {
-		/*
-		 * There is no unmerged entry, don't advise 'git
-		 * add/rm <file>', just 'git commit'.
-		 */
-		if (advice_resolve_conflict)
-			die("You have not concluded your merge (MERGE_HEAD exists).\n"
-			    "Please, commit your changes before you can merge.");
-		else
-			die("You have not concluded your merge (MERGE_HEAD exists).");
-	}
-
-	resolve_undo_clear();
 	/*
 	 * Check if we are _not_ on a detached HEAD, i.e. if there is a
 	 * current branch.
@@ -929,6 +913,23 @@
 
 	argc = parse_options(argc, argv, prefix, builtin_merge_options,
 			builtin_merge_usage, 0);
+
+	if (read_cache_unmerged())
+		die_resolve_conflict("merge");
+
+	if (file_exists(git_path("MERGE_HEAD"))) {
+		/*
+		 * There is no unmerged entry, don't advise 'git
+		 * add/rm <file>', just 'git commit'.
+		 */
+		if (advice_resolve_conflict)
+			die("You have not concluded your merge (MERGE_HEAD exists).\n"
+			    "Please, commit your changes before you can merge.");
+		else
+			die("You have not concluded your merge (MERGE_HEAD exists).");
+	}
+	resolve_undo_clear();
+
 	if (verbosity < 0)
 		show_diffstat = 0;