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>

Author: chris3torek

builtin/mv.c

@@ -220,9 +220,18 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 				}
 				argc += last - first;
 			}
-		} else if (cache_name_pos(src, length) < 0)
-			bad = _("not under version control");
-		else if (lstat(dst, &st) == 0 &&
+		} else if (cache_name_pos(src, length) < 0) {
+			/*
+			 * This occurs for both untracked files *and*
+			 * files that are in merge-conflict state, so
+			 * let's distinguish between those two.
+			 */
+			struct cache_entry *ce = cache_file_exists(src, length, ignore_case);
+			if (ce == NULL)
+				bad = _("not under version control");
+			else
+				bad = _("must resolve merge conflict first");
+		} else if (lstat(dst, &st) == 0 &&
 			 (!ignore_case || strcasecmp(src, dst))) {
 			bad = _("destination exists");
 			if (force) {

t/t7001-mv.sh

@@ -248,6 +248,24 @@ 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 &&
+	touch conflicted &&
+	cfhash=$(git hash-object -w conflicted) &&
+	git update-index --index-info <<-EOF &&
+	$(printf "0 $cfhash 0\tconflicted\n")
+	$(printf "100644 $cfhash 1\tconflicted\n")
+	EOF
+
+	test_must_fail git mv conflicted newname 2>actual &&
+	test_i18ngrep "merge.conflict" actual
+'
+
+rm -f conflicted
+
 test_expect_success 'git mv should overwrite symlink to a file' '
 
 	rm -fr .git &&