Merge branch 'mm/subprocess-handshake-fix'

The subprocess handshake during startup has been made gentler by using
packet_read_line_gently() instead of packet_read_line() to prevent the
parent Git process from dying abruptly when a configured subprocess
(e.g., a clean/smudge filter) fails to start.

* mm/subprocess-handshake-fix:
  sub-process: use gentle handshake to avoid die() on startup failure