Apply mirror git config for serving performance

[alecthomas]

After switching to --mirror clones (#123), apply the git config settings from the design doc (#23) to optimize upload-pack serving.

Changes required

internal/gitclone/manager.go

Add a configureMirror() helper called after git clone --mirror succeeds in executeClone(). It should run git config for each of:

Protocol:

• protocol.version 2
• uploadpack.allowFilter true (already set, move into this helper)
• uploadpack.allowReachableSHA1InWant true

Bitmaps (biggest win for upload-pack):

• repack.writeBitmaps true
• pack.useBitmaps true
• pack.useBitmapBoundaryTraversal true

Commit graph (no --changed-paths; Bloom filters do not help upload-pack):

• core.commitGraph true
• gc.writeCommitGraph true
• fetch.writeCommitGraph true

Multi-pack-index (avoids full repack on every fetch):

• core.multiPackIndex true

Never unpack loose — keep fetched objects as packs:

• transfer.unpackLimit 1
• fetch.unpackLimit 1

Disable auto GC — maintenance is explicit (see #125):

• gc.auto 0

Pack performance:

• pack.threads 0
• pack.deltaCacheSize 512m
• pack.windowMemory 1g

Existing repos

Also call configureMirror() for repos discovered by DiscoverExisting() on startup, so repos cloned before these settings existed get updated.

Tests

• Test that after clone, the expected git config values are set (read them back with git config --get).