[PATCH v3 2/3] read-tree -m: make error message for merging 0 trees less smart aleck

[Jean-Noel Avila <jn.avila@free.fr>]

"git read-tree -m" requires a tree argument to name the tree to be
merged in.  Git uses a cutesy error message to say so and why:

    $ git read-tree -m
    warning: read-tree: emptying the index with no arguments is
    deprecated; use --empty
    fatal: just how do you expect me to merge 0 trees?
    $ git read-tree -m --empty
    fatal: just how do you expect me to merge 0 trees?

When lucky, that could produce an ah-hah moment for the user, but it's
more likely to irritate and distract them.

Instead, tell the user plainly that the tree argument is
required. Also document this requirement in the git-read-tree(1)
manpage where there is room to explain it in a more straightforward way.

Signed-off-by: Jean-Noel Avila <jn.avila@free.fr>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
---
 Documentation/git-read-tree.txt | 8 ++++----
 builtin/read-tree.c             | 7 ++++---
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/Documentation/git-read-tree.txt b/Documentation/git-read-tree.txt
index ed9d63ef4..97df00043 100644
--- a/Documentation/git-read-tree.txt
+++ b/Documentation/git-read-tree.txt
@@ -135,10 +135,10 @@ OPTIONS
 
 Merging
 -------
-If `-m` is specified, 'git read-tree' can perform 3 kinds of
-merge, a single tree merge if only 1 tree is given, a
-fast-forward merge with 2 trees, or a 3-way merge if 3 trees are
-provided.
+If `-m` is specified, at least one tree must be given on the command
+line. 'git read-tree' can perform 3 kinds of merge, a single tree
+merge if only 1 tree is given, a fast-forward merge with 2 trees, or a
+3-way merge if 3 or more trees are provided.
 
 
 Single Tree Merge
diff --git a/builtin/read-tree.c b/builtin/read-tree.c
index 23e212ee8..de1a58d17 100644
--- a/builtin/read-tree.c
+++ b/builtin/read-tree.c
@@ -132,7 +132,7 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 		OPT_BOOL(0, "empty", &read_empty,
 			    N_("only empty the index")),
 		OPT__VERBOSE(&opts.verbose_update, N_("be verbose")),
-		OPT_GROUP(N_("Merging")),
+		OPT_GROUP(N_("Merging (needs at least one tree-ish")),
 		OPT_BOOL('m', NULL, &opts.merge,
 			 N_("perform a merge in addition to a read")),
 		OPT_BOOL(0, "trivial", &opts.trivial_merges_only,
@@ -226,9 +226,10 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 		setup_work_tree();
 
 	if (opts.merge) {
-		if (stage < 2)
-			die("just how do you expect me to merge %d trees?", stage-1);
 		switch (stage - 1) {
+		case 0:
+			die(_("you must specify at least one tree to merge"));
+			break;
 		case 1:
 			opts.fn = opts.prefix ? bind_merge : oneway_merge;
 			break;
-- 
2.13.0