gogit: add Git-compatible config get, set, and unset

[MuskanPaliwal]

Closes #12

Add Git-compatible configuration inspection and mutation through both modern subcommands and established legacy spellings:

• gogit config get <key>
• gogit config set <key> <value>
• gogit config unset <key>
• gogit config --get, --get-all, --add, --replace-all, --unset, and --unset-all
• bare gogit config <key> [<value>]

The implementation reads the effective configuration hierarchy while directing writes to one physical configuration file. It preserves comments, formatting, repeated sections, key spelling, symlink targets, and Git-compatible exit behavior.

Why configuration needs two different models

A configuration read is a merged view assembled from system, global, repository, worktree, included-file, and command-line sources. A write must instead mutate exactly one physical file.

The implementation keeps these operations separate:

• reads collect sources in ascending precedence and return the last matching value unless --all is requested;
• writes resolve one target according to --file, --local, --global, or --system, defaulting to the repository configuration;
• --global reads both XDG and ~/.gitconfig, while writes prefer an existing ~/.gitconfig, then an existing XDG file, and otherwise create ~/.gitconfig;
• linked worktrees read config.worktree when extensions.worktreeConfig enables it, while ordinary local writes continue to target the common repository configuration.

Default effective reads follow Git include directives at their original position in the file. This supports unconditional includes and the gitdir, gitdir/i, onbranch, and hasconfig:remote.*.url conditions. Explicit single-scope and --file reads leave includes disabled, matching Git’s default include behavior for those modes.

Format-preserving configuration editing

The new internal configuration representation retains the source structure rather than decoding and regenerating the entire file. Mutations can therefore change one option without discarding unrelated comments, blank lines, quoting, casing, repeated sections, or same-line section declarations.

Ordered entries also distinguish a valueless option from an explicitly empty value. That distinction is required for Git boolean semantics, where a bare option means true but option = does not.

Configuration keys are parsed and matched case-insensitively while preserving the spelling supplied by the caller when a new key is written.

Safe concurrent writes

Configuration mutation uses Git-style <config>.lock files created exclusively. The lock covers the complete read-modify-write transaction:

1. resolve the real target when the configuration path is a symlink;
2. acquire the exclusive lock;
3. read the latest target contents;
4. apply the mutation;
5. preserve an existing file’s permissions;
6. flush and atomically rename the lock over the target.

This prevents two successful writers from committing stale snapshots over one another. A contending writer fails visibly instead of reporting success while silently losing another update.

New files use normal 0666 & umask creation semantics. Existing files retain their permission bits.

Git-compatible behavior

The command supports configuration precedence, repeated values, scope selection, linked worktrees, bare repositories, -c overrides, path expansion, and Git-specific exit statuses.

--path expands both the current user forms (~ and ~/...) and existing named users (~user/...). GIT_CONFIG_NOSYSTEM uses Git-style boolean parsing, so values such as 0, false, no, and off do not disable the system scope.

The literal config --set spelling is deliberately not added: GNU Git rejects that flag. The supported forms are the modern config set <key> <value> command and the legacy bare config <key> <value> form.

Verification

The following checks pass:

GOCACHE=/tmp/gogit-review-cache go test ./...
GOCACHE=/tmp/gogit-review-cache go test -race ./internal/plumbing/format/config ./cmd/gogit
GOCACHE=/tmp/gogit-review-cache go vet ./...
GOCACHE=/tmp/gogit-review-cache GOLANGCI_LINT_CACHE=/tmp/gogit-review-lint-cache build/tools/golangci-lint-v2.12.2 run ./...
git diff --check

Focused comparisons against GNU Git also verified:

• a new configuration file created under umask 077 has mode 0600;
• parallel writers never silently lose a successfully reported mutation;
• included configuration values participate in effective lookup;
• worktree configuration overrides common repository configuration;
• ~/.gitconfig is the explicit global write target when both global files exist;
• GIT_CONFIG_NOSYSTEM=false leaves system configuration enabled;
• ~user/path expands to the named user’s home directory.