1. This is a combination of 8 commits.
2. This is the 1st commit message:

Support linked working directories

Support for linked working directories:

• Add commondir() (equivalent to GIT_COMMON_DIR)
• Read the commondir file, to set it.

See git-worktree(1) and gitrepository-layout(5).

The commit message #2 will be skipped:

# Fix DiskRefsContainer.refpath()

The commit message #3 will be skipped:

# Add a testsuite

The commit message #4 will be skipped:

# Add @skipIf for WorkingTreeTestCase

The commit message #5 will be skipped:

# worktree is optional and default to path

The commit message #6 will be skipped:

# add a TODO

The commit message #7 will be skipped:

1. Save one syscall #
2. See https://github.com/jelmer/dulwich/pull/454/files/0927deb7cd2ad24294b89e319ea060ed488acbba#r82424872

The commit message #8 will be skipped:

1. Read commondir with get_named_file()