dim: Share git object storage

Apparently this needs an explicit repack to work, which reduces risks
with pruning. So do this only in the setup functions.

Prep work for multiple workdir checkouts, but already saves an
impressive 3G here.

dim

@@ -463,6 +463,8 @@ function setup_dim
 	fi
 	cd maintainer-tools
 	git config remote.origin.url $drm_intel_ssh
+	echo "$DIM_PREFIX/$DIM_DRM_INTEL/.git/objects" > .git/objects/info/alternates
+	git repack -a -d -l
 	if ! git branch | grep maintainer-tools > /dev/null ; then
 		git checkout -t origin/maintainer-tools
 	fi
@@ -473,6 +475,8 @@ function setup_dim
 	fi
 	cd drm-intel-rerere
 	git config remote.origin.url $drm_intel_ssh
+	echo "$DIM_PREFIX/$DIM_DRM_INTEL/.git/objects" > .git/objects/info/alternates
+	git repack -a -d -l
 	if ! git branch | grep rerere-cache > /dev/null ; then
 		git checkout -t origin/rerere-cache
 	fi
@@ -484,6 +488,8 @@ function setup_dim
 	cd drm-intel-nightly
 	mkdir -p .git/rr-cache
 	git config remote.origin.url $drm_intel_ssh
+	echo "$DIM_PREFIX/$DIM_DRM_INTEL/.git/objects" > .git/objects/info/alternates
+	git repack -a -d -l
 	if ! git branch | grep drm-intel-nightly > /dev/null ; then
 		git checkout -t origin/drm-intel-nightly
 	fi