GIT_ONE_FILESYSTEM: flip the default to stop at filesystem boundaries

Regarding the new environment variable, Linus Torvalds
<torvalds@linux-foundation.org> writes on Tue, 30 Mar 2010 in
<alpine.LFD.2.00.1003301537150.3707@i5.linux-foundation.org>:

    I suspect that it is _very_ unusual to have a source repo that crosses
    multiple filesystems, and the original reason for this patch-series
    seems to me to be likely to be more common than that multi-fs case. So
    having the logic go the other way would seem to match the common case,
    no?

The "crossing filesystem boundary" condition is checked by comparing
st_dev field in the result from stat(2).  This is slightly worrysome if
non-POSIX ports return different values in the field even for directories
in the same work tree extracted to the same "filesystem".  Erik Faye-Lund
confirms that in the msysgit port st_dev is 0, so this should be safe, as
"even Windows is safe" ;-)

This will affect those who use /.git to cram /etc and /home/me in the same
repostiory, /home is mounted from non-root filesystem, and a git operation
is done from inside /home/me/src.  But that is such a corner case we don't
want to give preference over helping people who will benefit from having
this default so that they do not have to suffer from slow automounters.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -531,10 +531,14 @@ git so take care if using Cogito etc.
 	(Useful for excluding slow-loading network directories.)
 
 'GIT_ONE_FILESYSTEM'::
-	If set to a true value ("true" or a non-zero integer), stop at
-	filesystem boundaries when looking for a repository directory.
-	Like 'GIT_CEILING_DIRECTORIES', it will not affect an explicit
-	respository directory set via 'GIT_DIR' or on the command line.
+	When run in a directory that does not have ".git" repository
+	directory, git tries to find such a directory in the parent
+	directories to find the top of the working tree, but by default it
+	does not cross filesystem boundaries.  This environment variable
+	can be set to false value ("false" or zero) to tell git not to
+	stop at filesystem boundaries.  Like 'GIT_CEILING_DIRECTORIES',
+	this will not affect an explicit respository directory set via
+	'GIT_DIR' or on the command line.
 
 git Commits
 ~~~~~~~~~~~

setup.c

@@ -323,7 +323,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 	const char *gitdirenv;
 	const char *gitfile_dir;
 	int len, offset, ceil_offset, root_len;
-	int current_device = 0, one_filesystem = 0;
+	int current_device = 0, one_filesystem = 1;
 	struct stat buf;
 
 	/*
@@ -444,7 +444,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 				}
 				cwd[offset] = '\0';
 				die("Not a git repository (or any parent up to mount parent %s)\n"
-					"Stopping at filesystem boundary since GIT_ONE_FILESYSTEM is set.", cwd);
+				"Stopping at filesystem boundary since GIT_ONE_FILESYSTEM is true.", cwd);
 			}
 		}
 		if (chdir("..")) {