Rename ONE_FILESYSTEM to DISCOVERY_ACROSS_FILESYSTEM

If a missing ONE_FILESYSTEM defaults to true, the only users who set this
variable set it to false to tell git not to limit the discovery to one
filesystem; there are too many negations in one sentence to make a simple
panda brain dizzy.

Use the variable GIT_DISCOVERY_ACROSS_FILESYSTEM that changes the
behaviour from the default "limit to one filesystem" to "cross the
boundary as I ask you to"; makes the semantics much more straight
forward.

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

Documentation/git.txt

@@ -530,15 +530,15 @@ git so take care if using Cogito etc.
 	a GIT_DIR set on the command line or in the environment.
 	(Useful for excluding slow-loading network directories.)
 
-'GIT_ONE_FILESYSTEM'::
+'GIT_DISCOVERY_ACROSS_FILESYSTEM'::
 	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.
+	can be set to true to tell git not to stop at filesystem
+	boundaries.  Like 'GIT_CEILING_DIRECTORIES', this will not affect
+	an explicit repository directory set via 'GIT_DIR' or on the
+	command line.
 
 git Commits
 ~~~~~~~~~~~

setup.c

@@ -392,7 +392,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 	 *   etc.
 	 */
 	offset = len = strlen(cwd);
-	one_filesystem = git_env_bool("GIT_ONE_FILESYSTEM", 0);
+	one_filesystem = !git_env_bool("GIT_DISCOVERY_ACROSS_FILESYSTEM", 0);
 	if (one_filesystem) {
 		if (stat(".", &buf))
 			die_errno("failed to stat '.'");
@@ -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 true.", cwd);
+				"Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).", cwd);
 			}
 		}
 		if (chdir("..")) {