commit-graph: Introduce `git_commit_graph_needs_refresh()`

commit-graph: Support lookups of entries in a commit-graph

merge: Check insert_head_ids error in create_virtual_base

Check git_signature_dup failure

insert_head_ids can fail due to allocation error

git_signature_dup can have an allocation failure

Fix some typos

include: fix typos in comments

Fix documentation formating on repository.h

The enum 'git_repository_init_flag_t', 'git_repository_init_mode_t' and
the structure 'git_repository_init_options' does not follow the format
used by docurium.

index: Check git_vector_dup error in write_entries

This is for extra safety within write_entries

If allocating case_sorted.contents fails, git_vector_sort will segfault.

To prevent a race condition, these wlock/rlock calls should probably be checked
for errors.

Invert the workflow conditionals so that a workflow_dispatch event acts
like an on: push build.

Add new bindings for the R language

Update .gitignore

patch: add owner accessor

commit-graph: Introduce a parser for commit-graph files

revspec: rename git_revparse_mode_t to git_revspec_t

mwindow: Fix a bug in the LRU window finding code

The information about the type of a revision spec is not information
about the parser.  Name it accordingly, so that `git_revparse_mode_t`
is now `git_revspec_t`.  Deprecate the old name.

Only exclude tags files/folders and build folders at the repository root.

Our test resources shouldn't count toward our language statistics.  We
have some PHP checked in to the test resources, but I assure you that we
do not have any production code in PHP and we shouldn't be blamed for
the test data.

This change now updates the `lru_window` variable to match the current
file's MRU window. This makes it such that it doesn't always choose the
file that happened to come last in the list of window files, and instead
should now correctly choose the file with the least-recently-used one.

ci: don't use ninja on macOS

Ninja is not installed by default on the macOS machines; stop trying to
use it.  Instead use `make -j` which should be roughly equivalent in
performance but supported everywhere.

midx: Fix a bug in `git_midx_needs_refresh()`

clone: set refs/remotes/origin/HEAD when branch is specified

This change introduces a function that allows the caller to know whether
the `commit-graph` file has not been modified since it was parsed.

Part of: #5757

This change introduces `git_commit_graph_entry_find()` and
`git_commit_graph_entry_parent()`. These two functions allow a much
faster lookup of commits by ID, since the ODB does not need to be
consulted, the commit object does not need to be inflated, and the
contents of the commit object do not need to be parsed.

Part of: #5757

This change is the first in a series to add support for git's
commit-graph. This should speed up commit graph traversals by avoiding
object parsing and allowing some operations to terminate earlier.

Part of: #5757

When a branch is specified to check out in clone, update the remote
tracking `HEAD` to point to it.  This mimics git's behavior, when
`git clone -b <name>` is used.

Use `p_pwrite`/`p_pread` consistently throughout the codebase

README: instructions for using libgit2 without compiling

Cope with empty default branch