write_file(): drop "fatal" parameter

All callers except three passed 1 for the "fatal" parameter to ask
this function to die upon error, but to a casual reader of the code,
it was not all obvious what that 1 meant.  Instead, split the
function into two based on a common write_file_v() that takes the
flag, introduce write_file_gently() as a new way to attempt creating
a file without dying on error, and make three callers to call it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -205,13 +205,13 @@ static int write_state_text(const struct am_state *state,
 		fmt = "%s\n";
 	else
 		fmt = "%s";
-	return write_file(am_path(state, name), 1, fmt, string);
+	return write_file(am_path(state, name), fmt, string);
 }
 
 static int write_state_count(const struct am_state *state,
 			     const char *name, int value)
 {
-	return write_file(am_path(state, name), 1, "%d\n", value);
+	return write_file(am_path(state, name), "%d\n", value);
 }
 
 static int write_state_bool(const struct am_state *state,