launch_editor(): indicate that Git waits for user input

When a graphical GIT_EDITOR is spawned by a Git command that opens
and waits for user input (e.g. "git rebase -i"), then the editor window
might be obscured by other windows. The user might be left staring at
the original Git terminal window without even realizing that s/he needs
to interact with another window before Git can proceed. To this user Git
appears hanging.

Print a message that Git is waiting for editor input in the original
terminal and get rid of it when the editor returns, if the terminal
supports erasing the last line.  Also, make sure that our message is
terminated with a whitespace so that any message the editor may show
upon starting up will be kept separate from our message.

Power users might not want to see this message or their editor might
already print such a message (e.g. emacsclient). Allow these users to
suppress the message by disabling the "advice.waitingForEditor" config.

The standard advise() function is not used here as it would always add
a newline which would make deleting the message harder.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

advice.h

@@ -19,6 +19,7 @@ extern int advice_set_upstream_failure;
 extern int advice_object_name_warning;
 extern int advice_rm_hints;
 extern int advice_add_embedded_repo;
+extern int advice_waiting_for_editor;
 
 int git_default_advice_config(const char *var, const char *value);
 __attribute__((format (printf, 1, 2)))