read_branches_file: plug a FILE* leak

read_branches_file: plug a FILE* leak

The earlier rewrite f28e3ab2 (read_branches_file: simplify string handling)
of read_branches_file() lost an fclose() call. Put it back.

As on Windows files that are open cannot be removed, the leak manifests in
a failure of 'git remote rename origin origin' when the remote's URL is
specified in .git/branches/origin, because by the time that the command
attempts to remove this file, it is still open.

Signed-off-by: Johannes Sixt <redacted>
Acked-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>