Merge branch 'tb/gc-recent-object-hook'

"git pack-objects" learned to invoke a new hook program that
enumerates extra objects to be used as anchoring points to keep
otherwise unreachable objects in cruft packs.

* tb/gc-recent-object-hook:
  gc: introduce `gc.recentObjectsHook`
  reachable.c: extract `obj_is_recent()`

Documentation/config/gc.txt

@@ -130,6 +130,21 @@ or rebase occurring.  Since these changes are not part of the current
 project most users will want to expire them sooner, which is why the
 default is more aggressive than `gc.reflogExpire`.
 
+gc.recentObjectsHook::
+	When considering whether or not to remove an object (either when
+	generating a cruft pack or storing unreachable objects as
+	loose), use the shell to execute the specified command(s).
+	Interpret their output as object IDs which Git will consider as
+	"recent", regardless of their age. By treating their mtimes as
+	"now", any objects (and their descendants) mentioned in the
+	output will be kept regardless of their true age.
++
+Output must contain exactly one hex object ID per line, and nothing
+else. Objects which cannot be found in the repository are ignored.
+Multiple hooks are supported, but all must exit successfully, else the
+operation (either generating a cruft pack or unpacking unreachable
+objects) will be halted.
+
 gc.rerereResolved::
 	Records of conflicted merge you resolved earlier are
 	kept for this many days when 'git rerere gc' is run.