'git merge' can be called without any arguments if merge.defaultToUpstream
is set. However, when merge.defaultToUpstream is not set, the user will be
presented the usage information as if he entered a command with a wrong
syntaxis. Ironically, the usage information confirms that no arguments are
mandatory.
This adds a proper error message telling the user why the command failed. As
a side-effect this can help the user in discovering the possibility to merge
with the upstream branch by setting merge.defaultToUpstream.
Signed-off-by: Vincent van Ravesteijn <vfr@lyx.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>