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/init-db.c

@@ -378,7 +378,7 @@ static void separate_git_dir(const char *git_dir)
 			die_errno(_("unable to move %s to %s"), src, git_dir);
 	}
 
-	write_file(git_link, 1, "gitdir: %s\n", git_dir);
+	write_file(git_link, "gitdir: %s\n", git_dir);
 }
 
 int init_db(const char *template_dir, unsigned int flags)