When we rewrite the configuration file due to any of its values
being modified, we call `config_refresh` to update the in-memory
representation of our config file backend. This is needlessly
wasteful though, as `config_refresh` will always open the on-disk
representation to reads the file contents while we already know
the complete file contents at this point in time as we have just
written it to disk.

Implement a new function `config_refresh_from_buffer` that will
refresh the backend's config entries from a buffer instead of
from the config file itself. Note that this will thus _not_
update the backend's timestamp, which will cause us to re-read
the buffer when performing a read operation on it. But this is
still an improvement as we now lazily re-read the contents, and
most importantly we will avoid constantly re-reading the contents
if we perform multiple write operations.

The following strace demonstrates this if we're re-writing a key
multiple times. It uses our config example with `config_set`
changed to update the file 10 times with different keys:

	$ strace lg2 config x.x z |& grep '^open.*config'
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/home/pks/.config/git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3

And now with the optimization of `config_refresh_from_buffer`:

	$ strace lg2 config x.x z |& grep '^open.*config'
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/home/pks/.config/git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4

As can be seen, this is quite a lot of `open` calls less.