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.

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

Set Host Header to match CONNECT authority target

Prior to this change, for CONNECT requests, the Host header was set to
the host and port of the target http proxy. However, per the rfc7230 for
HTTP/1.1 this is incorrect as the Host header should match the target of
the CONNECT request, as detailed in section 5.3.3 & 5.4.

  5.3.3.  authority-form

   The authority-form of request-target is only used for CONNECT
   requests (Section 4.3.6 of [RFC7231]).

     authority-form = authority

   When making a CONNECT request to establish a tunnel through one or
   more proxies, a client MUST send only the target URI's authority
   component (excluding any userinfo and its "@" delimiter) as the
   request-target.  For example,

     CONNECT www.example.com:80 HTTP/1.1

  5.4.  Host

   <snip>

   A client MUST send a Host header field in all HTTP/1.1 request
   messages.  If the target URI includes an authority component, then a
   client MUST send a field-value for Host that is identical to that
   authority component, excluding any userinfo subcomponent and its "@"
   delimiter (Section 2.7.1).  If the authority component is missing or
   undefined for the target URI, then a client MUST send a Host header
   field with an empty field-value.

This issue was noticed when proxying requests through HAProxy 2.2 which
rejects these invalid http requests.

Fixes from code analysis