Add shallow clone support (--depth)

[pstreef]

Problem

When cloning repositories with many refs (250K+ tags/branches), JGit sends "have" lines for all local objects during fetch negotiation. This creates massive request payloads that cause RSocket fragmentation issues.

Solution

Add shallow clone support (--depth 1) which:

• Only fetches the tip commit(s)
• Sends zero "have" lines on initial clone (no local objects)
• Keeps subsequent fetch payloads tiny (only 1-2 commits to report)

API

Git.cloneRepository()
    .setURI("https://github.com/example/repo.git")
    .setDepth(1)
    .call();

Changes

File	Description
Transport.java	Add depth, deepenSince, deepenNots fields with accessors
CloneCommand.java	Add setDepth(), setShallowSince(), addShallowExclude()
FetchCommand.java	Add setDepth(), setShallowSince(), addShallowExclude()
BasePackFetchConnection.java	Send deepen commands, handle shallow-info response, write .git/shallow
FetchProcess.java	Skip completeness check for shallow clones
GitProtocolConstants.java	Add SECTION_SHALLOW_INFO constant

Testing

Tested manually against GitHub:

Clone completed in 2921ms
HEAD ref: 4b235690d4ecc37b3ebedc0fe52235ab95eaff11
src/ exists: true
Shallow file exists: true
SUCCESS: Shallow clone worked!

Notes

This is a simplified implementation compared to official JGit 6.x:

• Supports initial shallow clone (primary use case)
• Does not support --unshallow or deepening existing shallow clones
• Writes directly to .git/shallow file (vs ObjectDatabase API)