git 2.1.2

This is a bugfix release, including the following changes:
* "git push" over HTTP transport had an artificial limit on number of
  refs that can be pushed imposed by the command line length.
* When receiving an invalid pack stream that records the same object
  twice, multiple threads got confused due to a race.
* An attempt to remove the entire tree in the "git fast-import" input
  stream caused it to misbehave.
* Reachability check (used in "git prune" and friends) did not add a
  detached HEAD as a starting point to traverse objects still in use.
* "git config --add section.var val" used to lose existing
  section.var whose value was an empty string.
* "git fsck" failed to report that it found corrupt objects via its
  exit status in some cases.