Merge branch 'maint'

* maint:
  git-bisect: make "start", "good" and "skip" succeed or fail atomically
  git-am: cope better with an empty Subject: line
  Ignore leading empty lines while summarizing merges
  bisect: squelch "fatal: ref HEAD not a symref" misleading message
  builtin-apply: Show a more descriptive error on failure when opening a patch
  Clarify documentation of git-cvsserver, particularly in relation to git-shell

builtin-apply.c

@@ -3117,7 +3117,7 @@ int cmd_apply(int argc, const char **argv, const char *unused_prefix)
 
 		fd = open(arg, O_RDONLY);
 		if (fd < 0)
-			usage(apply_usage);
+			die("can't open patch '%s': %s", arg, strerror(errno));
 		read_stdin = 0;
 		set_default_whitespace_mode(whitespace_option);
 		errs |= apply_patch(fd, arg, inaccurate_eof);