The extension matching is case insensitive, so keep references to them
in lower case format, as that is what is used during the matching
process.

Signed-off-by: Paulo Gomes <pjbgf@linux.com>

EnableDotGitCommonDir was an opt-in flag to detect and honor the
commondir mechanism used by Git linked worktrees.

Requiring callers to explicitly enable this behavior is both surprising
and error-prone. Consumers of PlainOpen should not need to know whether
a repository is a primary working tree or a linked worktree, nor whether
it relies on a shared common directory.

This distinction directly impacts core repository semantics, including
object resolution and configuration lookup. Leaving it opt-in can lead
to subtle and inconsistent behavior.

Making commondir detection unconditional, abstracting this complexity
away from callers, ensures a consistent and predictable repository
handling which aligns with upstream Git.

Signed-off-by: Paulo Gomes <pjbgf@linux.com>

Signed-off-by: Paulo Gomes <pjbgf@linux.com>

Previous to #1850, go-git was oblivious to Git extensions, which
meant that at times it would try to handle repositories where the
underlying feature was not fully supported.

An example of that being worktreeConfig, meaning that any worktree
specific config were being ignored.

This commit adds that support to storage/filesystem, resulting into
worktree configs being overlayed the local config, providing upstream
parity with upstream Git.

References:
- https://git-scm.com/docs/git-worktree#_configuration_file

Signed-off-by: Paulo Gomes <pjbgf@linux.com>

The internal merge helper had no case for reflect.Map, so map-typed
fields (Remotes, Branches, Submodules, URLs) fell through to the default
branch which called df.Set(sf), replacing the destination map wholesale.

This caused two bugs:

* An empty but non-nil source map (as produced by NewConfig) would
  overwrite a populated destination map, silently discarding all
  accumulated entries.

* A non-empty source map would replace the destination map entirely,
  dropping any destination keys absent from the source instead of
  merging them together.

Add a reflect.Map case that skips empty source maps unchanged and
otherwise copies entries one-by-one, so destination keys not present
in the source are preserved and source entries override same-key
destination entries.

Add table-driven tests covering nil source map, empty source map,
disjoint key sets, and same-key override.

Signed-off-by: Paulo Gomes <paulo@entire.io>

Signed-off-by: Paulo Gomes <pjbgf@linux.com>

When the worktreeConfig extension is active and a config.worktree file
already exists, only the delta — options whose values are absent from or
differ from the base config — is written to config.worktree. The base
config is left untouched in that case.

This mirrors the behaviour of 'git config --worktree' to some extend, and
enable the use case whereby worktree-specific overrides live in config.worktree
while shared settings remain in the common config.

Signed-off-by: Paulo Gomes <paulo@entire.io>