git-mv: improve error message for conflicted file

'git mv' has always complained about renaming a conflicted
file, as it cannot handle multiple index entries for one file.
However, the error message it uses has been the same as the
one for an untracked file:

    fatal: not under version control, src=...

which is patently wrong.  Distinguish the two cases and
add a test to make sure we produce the correct message.

Signed-off-by: Chris Torek <chris.torek@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/mv.c

@@ -132,6 +132,7 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 	struct stat st;
 	struct string_list src_for_dst = STRING_LIST_INIT_NODUP;
 	struct lock_file lock_file = LOCK_INIT;
+	struct cache_entry *ce;
 
 	git_config(git_default_config, NULL);
 
@@ -220,9 +221,11 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 				}
 				argc += last - first;
 			}
-		} else if (cache_name_pos(src, length) < 0)
+		} else if (!(ce = cache_file_exists(src, length, ignore_case))) {
 			bad = _("not under version control");
-		else if (lstat(dst, &st) == 0 &&
+		} else if (ce_stage(ce)) {
+			bad = _("conflicted");
+		} else if (lstat(dst, &st) == 0 &&
 			 (!ignore_case || strcasecmp(src, dst))) {
 			bad = _("destination exists");
 			if (force) {

t/t7001-mv.sh

@@ -248,6 +248,23 @@ test_expect_success 'git mv should not change sha1 of moved cache entry' '
 
 rm -f dirty dirty2
 
+# NB: This test is about the error message
+# as well as the failure.
+test_expect_success 'git mv error on conflicted file' '
+	rm -fr .git &&
+	git init &&
+	>conflict &&
+	test_when_finished "rm -f conflict" &&
+	cfhash=$(git hash-object -w conflict) &&
+	q_to_tab <<-EOF | git update-index --index-info &&
+	0 $cfhash 0Qconflict
+	100644 $cfhash 1Qconflict
+	EOF
+
+	test_must_fail git mv conflict newname 2>actual &&
+	test_i18ngrep "conflicted" actual
+'
+
 test_expect_success 'git mv should overwrite symlink to a file' '
 
 	rm -fr .git &&