Merge branch 'nd/checkout-disambiguation' into maint

"git checkout <word>" does not follow the usual disambiguation
rules when the <word> can be both a rev and a path, to allow
checking out a branch 'foo' in a project that happens to have a
file 'foo' in the working tree without having to disambiguate.
This was poorly documented and the check was incorrect when the
command was run from a subdirectory.

* nd/checkout-disambiguation:
  checkout: fix ambiguity check in subdir
  checkout.txt: document a common case that ignores ambiguation rules
  checkout: add some spaces between code and comment

Documentation/git-checkout.txt

@@ -419,6 +419,18 @@ $ git reflog -2 HEAD # or
 $ git log -g -2 HEAD
 ------------
 
+ARGUMENT DISAMBIGUATION
+-----------------------
+
+When there is only one argument given and it is not `--` (e.g. "git
+checkout abc"), and when the argument is both a valid `<tree-ish>`
+(e.g. a branch "abc" exists) and a valid `<pathspec>` (e.g. a file
+or a directory whose name is "abc" exists), Git would usually ask
+you to disambiguate.  Because checking out a branch is so common an
+operation, however, "git checkout abc" takes "abc" as a `<tree-ish>`
+in such a situation.  Use `git checkout -- <pathspec>` if you want
+to checkout these paths out of the index.
+
 EXAMPLES
 --------
 

builtin/checkout.c

@@ -985,7 +985,7 @@ static int parse_branchname_arg(int argc, const char **argv,
 		int recover_with_dwim = dwim_new_local_branch_ok;
 
 		if (!has_dash_dash &&
-		    (check_filename(NULL, arg) || !no_wildcard(arg)))
+		    (check_filename(opts->prefix, arg) || !no_wildcard(arg)))
 			recover_with_dwim = 0;
 		/*
 		 * Accept "git checkout foo" and "git checkout foo --"
@@ -1038,7 +1038,7 @@ static int parse_branchname_arg(int argc, const char **argv,
 
 	if (!*source_tree)                   /* case (1): want a tree */
 		die(_("reference is not a tree: %s"), arg);
-	if (!has_dash_dash) {/* case (3).(d) -> (1) */
+	if (!has_dash_dash) {	/* case (3).(d) -> (1) */
 		/*
 		 * Do not complain the most common case
 		 *	git checkout branch
@@ -1046,7 +1046,7 @@ static int parse_branchname_arg(int argc, const char **argv,
 		 * it would be extremely annoying.
 		 */
 		if (argc)
-			verify_non_filename(NULL, arg);
+			verify_non_filename(opts->prefix, arg);
 	} else {
 		argcount++;
 		argv++;

t/t2010-checkout-ambiguous.sh

@@ -41,6 +41,15 @@ test_expect_success 'check ambiguity' '
 	test_must_fail git checkout world all
 '
 
+test_expect_success 'check ambiguity in subdir' '
+	mkdir sub &&
+	# not ambiguous because sub/world does not exist
+	git -C sub checkout world ../all &&
+	echo hello >sub/world &&
+	# ambiguous because sub/world does exist
+	test_must_fail git -C sub checkout world ../all
+'
+
 test_expect_success 'disambiguate checking out from a tree-ish' '
 	echo bye > world &&
 	git checkout world -- world &&

t/t2024-checkout-dwim.sh

@@ -174,6 +174,18 @@ test_expect_success 'checkout of branch with a file having the same name fails'
 	test_branch master
 '
 
+test_expect_success 'checkout of branch with a file in subdir having the same name fails' '
+	git checkout -B master &&
+	test_might_fail git branch -D spam &&
+
+	>spam &&
+	mkdir sub &&
+	mv spam sub/spam &&
+	test_must_fail git -C sub checkout spam &&
+	test_must_fail git rev-parse --verify refs/heads/spam &&
+	test_branch master
+'
+
 test_expect_success 'checkout <branch> -- succeeds, even if a file with the same name exists' '
 	git checkout -B master &&
 	test_might_fail git branch -D spam &&