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>

setup.c

@@ -404,7 +404,7 @@ static void update_linked_gitdir(const char *gitfile, const char *gitdir)
 
 	strbuf_addf(&path, "%s/gitfile", gitdir);
 	if (stat(path.buf, &st) || st.st_mtime + 24 * 3600 < time(NULL))
-		write_file(path.buf, 0, "%s\n", gitfile);
+		write_file_gently(path.buf, "%s\n", gitfile);
 	strbuf_release(&path);
 }