git: bump to version 1.8.5.2

Fixes since v1.8.5.1:
 * "git diff -- ':(icase)makefile'" was unnecessarily rejected at the
   command line parser.

 * "git cat-file --batch-check=ok" did not check the existence of
   the named object.

 * "git am --abort" sometimes complained about not being able to write
   a tree with an 0{40} object in it.

 * Two processes creating loose objects at the same time could have
   failed unnecessarily when the name of their new objects started
   with the same byte value, due to a race condition.

Also contains typofixes, documentation updates and trivial code clean-ups.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>