trailer: ensure we identify patch lines like git

Git looks explicitly for `---` followed by whitespace (`isspace()`) to
determine when a patch line begins in a commit message. Match that
behavior. This ensures that we don't treat (say) `----` as a patch line
incorrectly.

src/libgit2/trailer.c

@@ -158,7 +158,7 @@ static size_t find_patch_start(const char *str)
 	const char *s;
 
 	for (s = str; *s; s = next_line(s)) {
-		if (git__prefixcmp(s, "---") == 0)
+		if (git__prefixcmp(s, "---") == 0 && git__isspace(s[3]))
 			return s - str;
 	}
 

tests/libgit2/message/trailer.c

@@ -165,3 +165,23 @@ void test_message_trailer__invalid(void)
 		"Another: trailer\n"
 	, trailers);
 }
+
+void test_message_trailer__ignores_dashes(void)
+{
+	git_message_trailer trailers[] = {
+		{ "Signed-off-by", "some@one.com" },
+		{ "Another", "trailer" },
+		{ NULL, NULL },
+	};
+
+	assert_trailers(
+		"Message\n"
+		"\n"
+		"Markdown header\n"
+		"---------------\n"
+		"Lorem ipsum\n"
+		"\n"
+		"Signed-off-by: some@one.com\n"
+		"Another: trailer\n"
+	, trailers);
+}