CMakeLists: Show the pointer size for an unsupported architecture

Showing the pointer size gives a hint as to why we think this is an
unsupported architecture.

When the array is empty `cmp` never gets set by the comparison
function. Initialize it so we return ENOTFOUND in those cases.

tree: drop the now-unnecessary entries vector

Add ability to write a filtered blob through a stream

The callback mechanism makes it awkward to write data from an IO
source; move to `_fromstream()` which lets the caller remain in control,
in the same vein as we prefer iterators over foreach callbacks.

By returning when the count goes to zero rather than below it, setting
`howmany` to 7 in fact writes out the string 6 times.

Correct the termination condition to write out the string the amount of
times we specify.

The pair of `git_blob_create_frombuffer()` and
`git_blob_create_frombuffer_commit()` is meant to replace
`git_blob_create_fromchunks()` by providing a way for a user to write a
new blob when they want filtering or they do not know the size.

This approach allows the caller to retain control over when to add data
to this buffer and a more natural fit into higher-level language's own
stream abstractions instead of having to handle IO wait in the callback.

The in-memory buffer size of 2MB is chosen somewhat arbitrarily to be a
round multiple of usual page sizes and a value where most blobs seem
likely to be either going to be way below or way over that size. It's
also a round number of pages.

This implementation re-uses the helper we have from `_fromchunks()` so
we end up writing everything to disk, but hopefully more efficiently
than with a default filebuf. A later optimisation can be to avoid
writing the in-memory contents to disk, with some extra complexity.

Allow setting the buffer size on open in order to use this data
structure more generally as a spill buffer, with larger buffer sizes for
specific use-cases.

Remove the now-unnecessary entries vector.  Add `git_array_search`
to binary search through an array to accomplish this.

Add a sanity check in git_indexer_commit to avoid subtraction overflow.

Fix some errors I found in the changelog for 0.24.0

Reuse a tree's buffer and allocate constant-sized entries in an array

Take advantage of the constant size of tree-owned arrays and store them
in an array instead of a pool. This still lets us free them all at once
but lets the system allocator do the work of fitting them in.

Instead of copying over the data into the individual entries, point to
the originals, which are already in a format we can use.

win32: free thread-local data on thread exit

MinGW builds should optionally create DLLs without "lib" prefix

Custom merge drivers and proper gitattributes `merge` handling

CONTRIBUTING: document the optional tests

Since the `apply` callback can defer, the `check` callback is not
necessary.  Removing the `check` callback further makes the `payload`
unnecessary along with the `cleanup` callback.

Ensure that setting the merge attribute forces the built-in default
`text` driver and does *not* honor the `merge.default` configuration
option.  Further ensure that unsetting the merge attribute forces
a conflict (the `binary` driver).

Allow merge users to configure a custom default merge driver via
`git_merge_options`.  Similarly, honor the `merge.default` configuration
option.

When a `check` or `apply` callback function returns `GIT_EMERGECONFLICT`
stop and product a conflict.

When a `check` or `apply` callback function returns `GIT_PASSTHROUGH`,
move on to the default merge driver.

Consumers can now register custom merged drivers with
`git_merge_driver_register`.  This allows consumers to support the
merge drivers, as configured in `.gitattributes`.  Consumers will be
asked to perform the file-level merge when a custom driver is
configured.

commit: add function to attach a signature to a commit