I make a local prototype of repo, after try I found that the different projects share project-objects directory but don't share pack files.
For example, I create 3 repo projects contain frameworks/base, and I got one more pack file for each project:
zhangclb@zhangclb2:~/.project-objects/platform/frameworks/base.git/objects/pack$ ll
总用量 1974620
drwxrwxr-x 2 zhangclb zhangclb 4096 7月 6 15:44 ./
drwxrwxr-x 4 zhangclb zhangclb 4096 7月 6 15:38 ../
-r--r--r-- 1 zhangclb zhangclb 934872 7月 6 15:44 pack-7b7fde4f2ca24daf410dd08689192da77c221602.idx
-r--r--r-- 1 zhangclb zhangclb 673053895 7月 6 15:43 pack-7b7fde4f2ca24daf410dd08689192da77c221602.pack
-r--r--r-- 1 zhangclb zhangclb 934872 7月 6 15:41 pack-9e7571aa255fb6d251d71846315102951093fe05.idx
-r--r--r-- 1 zhangclb zhangclb 673055061 7月 6 15:41 pack-9e7571aa255fb6d251d71846315102951093fe05.pack
-r--r--r-- 1 zhangclb zhangclb 934872 7月 6 15:39 pack-ea3d70f588283d0be6b0d778251478e26b49fff8.idx
-r--r--r-- 1 zhangclb zhangclb 673059851 7月 6 15:39 pack-ea3d70f588283d0be6b0d778251478e26b49fff8.pack