builtin/maintenance: fix locking with "--detach"

When running git-maintenance(1), we create a lockfile that is supposed
to keep other maintenance processes from running at the same time. This
lockfile is broken though in case the "--detach" flag is passed: the
lockfile is created by the parent process and will be cleaned up either
manually or on exit. But when detaching, the parent will exit before all
of the background maintenance tasks have been run, and consequently the
lock only covers a smaller part of the whole maintenance process.

Fix this bug by reassigning all tempfiles from the parent process to the
child process when daemonizing so that it becomes the responsibility of
the child to clean them up.

Note that this is a broader fix, as we now always reassign tempfiles
when daemonizing. This is a natural consequence of the semantics of
`daemonize()` though, as it essentially promises to continue running the
current process in the background. It is thus sensible to have that
function perform the whole dance of assigning resources to the child
process, including tempfiles.

There's only a single other caller in "daemon.c", but that process
doesn't create any tempfiles before the call to `daemonize()` and is
thus not impacted by this change.

Reported-by: Jean-Christophe Manciot <actionmystique@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Helped-by: Derrick Stolee <stolee@gmail.com>
Co-authored-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.h

@@ -149,6 +149,21 @@ void verify_non_filename(const char *prefix, const char *name);
 int path_inside_repo(const char *prefix, const char *path);
 
 void sanitize_stdfds(void);
+
+/*
+ * Daemonize the current process by forking and then exiting the parent
+ * process. Returns 0 when successful, in which case the parent process will
+ * have exited and it's the child process that continues to run the code.
+ * Otherwise, a negative error code is returned and the parent process will
+ * continue execution.
+ *
+ * Note that this function will also perform the following changes:
+ *
+ *   - Standard file descriptors in the child process are closed.
+ *   - The child process is made a session leader via setsid(3p).
+ *   - All tempfiles owned by the parent process are reassigned to the
+ *     daemonized child process.
+ */
 int daemonize(void);
 
 /*