repack: optionally spawn pack-aggregate to keep objects/packs tidy

On a busy server hosting a large repository, `git repack` routinely
takes long enough for thousands of packs and tens of thousands of loose
objects to accumulate before it finishes.  Every subsequent git
operation pays the price: each pack lookup walks the longer list, and
each loose-object miss has to consult more sources.

Wire `git repack` to optionally drive `git pack-aggregate` as a
background side process, controlled by `repack.aggregate` config
or the equivalent `--aggregate` / `--no-aggregate` command-line
options.  Default off.

When enabled:

  * Create a small tempdir under `objects/` for two exclude files.
  * Pass `--emit-input-packs=<tmp>/packs --emit-input-loose=<tmp>/loose`
    to the main pack-objects so it records exactly which inputs it
    has committed to.
  * Launch `git pack-aggregate --loop --exclude-pack-file=<tmp>/packs
    --exclude-loose-file=<tmp>/loose` once the emit files exist.
  * Drop a `.keep` marker on every pack written by the main repacking
    work, so the aggregator does not roll it up.

pack-aggregate keeps running through the cruft pack-objects subprocess
and through `write_midx_included_packs()`; it is SIGTERM'd only when it
comes time to delete redundant packs and objects.

Finally, unlink the recorded `.keep` markers after pack-aggregate has
been shut down.

Add a few environment variables to simplify testing:
  * `GIT_TEST_PACK_AGGREGATE_INTERVAL`
  * `GIT_TEST_PACK_AGGREGATE_MIN_PACKS`
  * `GIT_TEST_PACK_AGGREGATE_MIN_LOOSE`

`git repack` itself takes no locks, and neither does the aggregator
we spawn; serialization across maintenance runs is the caller's
responsibility (typically `git gc` or a server-side maintenance
driver).  This matches the prior behavior of `git repack`.

Assisted-by: Claude Opus 4.7
Signed-off-by: Elijah Newren <newren@gmail.com>

Author: newren

Documentation/config/repack.adoc

@@ -64,3 +64,13 @@ repack.midxNewLayerThreshold::
 When the tip layer has fewer packs than this threshold, those packs are
 excluded from the geometric repack entirely, and are thus left
 unmodified. Must be at least 1. Defaults to 8.
+
+repack.aggregate::
+	If set to true, linkgit:git-repack[1] will spawn a background
+	linkgit:git-pack-aggregate[1] for the duration of its main
+	pack-objects run.  The aggregator rolls up small packs and
+	loose objects that arrive after pack-objects has enumerated
+	its inputs, preventing them from accumulating in
+	`objects/pack/` and slowing other Git operations on busy
+	servers.  Defaults to false.  Can be overridden on the
+	command line with `--aggregate` or `--no-aggregate`.

Documentation/git-repack.adoc

@@ -12,6 +12,7 @@ SYNOPSIS
 'git repack' [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [-m]
 	[--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>]
 	[--write-midx[=<mode>]] [--name-hash-version=<n>] [--path-walk]
+	[--[no-]aggregate]
 
 DESCRIPTION
 -----------
@@ -300,6 +301,16 @@ created for any new pack(s) without disturbing the existing chain.
 	Pass the `--path-walk` option to the underlying `git pack-objects`
 	process. See linkgit:git-pack-objects[1] for full details.
 
+--aggregate::
+--no-aggregate::
+	Spawn a background linkgit:git-pack-aggregate[1] for the
+	duration of the main pack-objects run.  The aggregator rolls
+	up small packs and loose objects that arrive after
+	pack-objects has enumerated its inputs, preventing them from
+	accumulating in `objects/pack/` and slowing other Git
+	operations on busy servers.  Overrides the
+	`repack.aggregate` configuration variable.  Off by default.
+
 CONFIGURATION
 -------------
 
@@ -324,6 +335,7 @@ SEE ALSO
 --------
 linkgit:git-pack-objects[1]
 linkgit:git-prune-packed[1]
+linkgit:git-pack-aggregate[1]
 
 GIT
 ---