Documentation: git gc packs refs by default now

Documentation: git gc packs refs by default now

In commit 56752391 (Make "git gc" pack all refs by default,
2007-05-24), 'git gc' was changed to run pack-refs by default

Versions before v1.5.1.2 cannot clone repos with packed refs over
http, and versions before v1.4.4 cannot handled packed refs at
all, but more recent git should have no problems.  Try to make
this more clear in the git-config manual.

The analagous passage in git-gc.txt was updated already with
commit fe2128a (Change git-gc documentation to reflect
gc.packrefs implementation., 2008-01-09).

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>