Implement core.symlinks to support filesystems without symlinks

Here is a small patch series that adds a configuration variable core.symlinks. From the manual:

core.symlinks::
	If false, symbolic links are checked out as small plain files that
	contain the link text. gitlink:git-update-index[1] and
	gitlink:git-add[1] will not change the recorded type to regular
	file. Useful on filesystems like FAT that do not support
	symbolic links. True by default.

The implementation was surprisingly simple:

 Documentation/config.txt           |    7 +++++++
 Documentation/git-update-index.txt |    5 +++++
 builtin-apply.c                    |    2 +-
 builtin-update-index.c             |    6 +++---
 cache.h                            |    7 ++++++-
 config.c                           |    5 +++++
 diff-lib.c                         |    3 +++
 entry.c                            |    5 +++++
 environment.c                      |    1 +
 read-cache.c                       |   12 +++++++-----
 10 files changed, 43 insertions(+), 10 deletions(-)

-- Hannes