Introduce a function to create an email from a diff and multiple inputs
about the source of the diff.

Creating an email from a diff requires many more inputs, and should be
discouraged in favor of building directly from a commit, and is thus in
the `sys` namespace.

Create `git_email_*` which will encapsulate email creation and
application, and `git_email_create_from_commit` in particular, which
creates an email for a single commit.

Use __typeof__ GNUC keyword for ISO C compatibility

indexer: Avoid one `mmap(2)`/`munmap(2)` pair per `git_indexer_append` call

Support custom git extensions

Fix coding style for pointer

Fix config parsing for multiline with multiple quoted comment chars

notes: use a buffer internally

Code cleanup to use `git_buf`s instead of simple c strings.

The strip_comments function uses the count of quotes to know if a
comment char (';' or '#') is the start of a comment or part of the
multiline as a string.
Unfortunately converting the count of quotes from previous lines to a
boolean meant that it would only work as expected in some cases
(0 quotes or an odd number of quotes).

This tests parsing a multiline string containing multiple quoted comment
chars. See #6019

Make some syntax change to follow coding style.

Turns out, double negatives are harder to parse than positive
statements.

It turns out that if we use `mmap(2)`, non-Windows remote filesystems
break due to permissions. If we don't, _Windows_ remote filesystems
break due to lack of coherence between memory mapped views of the file
and direct I/O operations done to the files.

To break out of this impossible situation, conditionally-compile
versions of Windows-specific `write_at` and `append_to_pack`.

Now that we're not using `mmap(2)` for writing stuff, we don't need to
truncate the file afterwards, since it'll have the correct size at the
end of the process. Whee~!

This change makes `append_to_pack` completely rely on `p_pwrite` to do
all its I/O instead of splitting it between `p_pwrite` and a
`mmap(2)`/`munmap(2)`+`memcpy(3)`. This saves a good chunk of user CPU
time and avoids making two syscalls per round, but doesn't really cut
down a lot of wall time (~1% on cloning the
[git](https://github.com/git/git.git) repository).

This change makes `git_remote_name_is_valid` be part of the public
interface of the library. This is needed for other language bindings to
be able to find this symbol (like in git2go, when linking against
libgit2 dynamically).

Allow users to specify additional repository extensions that they want
to support.  For example, callers can specify that they support
`preciousObjects` and then may open repositories that support
`extensions.preciousObjects`.

Similarly, callers may opt out of supporting extensions that the library
itself supports.

#6028: Check if `threadstate->error_t.message` is not `git_buf__initbuf` before freeing.

This follows the same principle as `buffer.c` where the same check is done before freeing the buffer. It fixes the crash described in #6028.

Update proxy configuration

Update the proxy detection for a remote.

1. Honor `http.<url>.proxy` syntax for a remote's direct URL and
   parent URLs.
2. Honor an empty configuration URL to override a proxy configuration.

Add tests to ensure that configuration specificity is honored.

Provide a utility method on a url to determine if it matches any pattern
in a comma-separated list, similar to what one would find in `NO_PROXY`
environment variables.

Provide a method to determine if a given URL matches a host:port pattern
like the ones found in `NO_PROXY` environment variables.

Introduce a utility function that compares a NUL terminated string to a
possibly not-NUL terminated string with length.  This is similar to
`strncmp` but with an added check to ensure that the lengths match (not
just the `size` portion of the two strings).

Item 2 of 3 from #4164

Signed-off-by: Mathieu Parent <math.parent@gmail.com>

Fix a possible null pointer dereference

This change fixes a possible null pointer dereference if anything inside
`git_commit_graph_writer_add_index_file` fails before the packfile being
valid.

https://scan6.coverity.com/reports.htm#v52218/p10377/fileInstanceId=122935896&defectInstanceId=32525576&mergedDefectId=1461634

Fix worktree iteration when repository has no common directory