Provide a mechanism to turn off symlink resolution in ceiling paths

Commit 1b77d83cab 'setup_git_directory_gently_1(): resolve symlinks
in ceiling paths' changed the setup code to resolve symlinks in the
entries in GIT_CEILING_DIRECTORIES.  Because those entries are
compared textually to the symlink-resolved current directory, an
entry in GIT_CEILING_DIRECTORIES that contained a symlink would have
no effect.  It was known that this could cause performance problems
if the symlink resolution *itself* touched slow filesystems, but it
was thought that such use cases would be unlikely.  The intention of
the earlier change was to deal with a case when the user has this:

	GIT_CEILING_DIRECTORIES=/home/gitster

but in reality, /home/gitster is a symbolic link to somewhere else,
e.g. /net/machine/home4/gitster. A textual comparison between the
specified value /home/gitster and the location getcwd(3) returns
would not help us, but readlink("/home/gitster") would still be
fast.

After this change was released, Anders Kaseorg <andersk@mit.edu>
reported:

> [...] my computer has been acting so slow when I’m not connected to
> the network.  I put various network filesystem paths in
> $GIT_CEILING_DIRECTORIES, such as
> /afs/athena.mit.edu/user/a/n/andersk (to avoid hitting its parents
> /afs/athena.mit.edu, /afs/athena.mit.edu/user/a, and
> /afs/athena.mit.edu/user/a/n which all live in different AFS
> volumes).  Now when I’m not connected to the network, every
> invocation of Git, including the __git_ps1 in my shell prompt, waits
> for AFS to timeout.

To allow users to work around this problem, give them a mechanism to
turn off symlink resolution in GIT_CEILING_DIRECTORIES entries.  All
the entries that follow an empty entry will not be checked for symbolic
links and used literally in comparison.  E.g. with these:

	GIT_CEILING_DIRECTORIES=:/foo/bar:/xyzzy or
	GIT_CEILING_DIRECTORIES=/foo/bar::/xyzzy

we will not readlink("/xyzzy") because it comes after an empty entry.

With the former (but not with the latter), "/foo/bar" comes after an
empty entry, and we will not readlink it, either.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Documentation/submodule: Add --force to update synopsis

In commit 9db31bdf (submodule: Add --force option for git submodule
update, 2011-04-01) we added the option to the implementation's usage
synopsis but forgot to add it to the synopsis in the command
documentation.  Add the option to the synopsis in the same location it
is reported in usage and re-wrap the options to avoid long lines.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'mh/maint-ceil-absolute'

An earlier workaround designed to help people who list logical
directories that will not match what getcwd(3) returns in the
GIT_CEILING_DIRECTORIES had an adverse effect when it is slow to
stat and readlink a directory component of an element listed on it.

* mh/maint-ceil-absolute:
  Provide a mechanism to turn off symlink resolution in ceiling paths
Merge branch 'jn/less-reconfigure' into maint

* jn/less-reconfigure:
  Makefile: avoid infinite loop on configure.ac change
Merge branch 'wk/man-deny-current-branch-is-default-these-days' into maint

* wk/man-deny-current-branch-is-default-these-days:
  user-manual: typofix (ofthe->of the)
  user-manual: Update for receive.denyCurrentBranch=refuse
Merge branch 'ef/non-ascii-parse-options-error-diag' into maint

* ef/non-ascii-parse-options-error-diag:
  parse-options: report uncorrupted multi-byte options
Update draft release notes to 1.8.1.5

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'maint'

* maint:
  Update draft release notes to 1.8.1.5
  Documentation/submodule: Add --force to update synopsis