git --bare cmd: do not unconditionally nuke GIT_DIR

"GIT_DIR=some.where git --bare cmd" and worse yet
"git --git-dir=some.where --bare cmd" were very confusing.  They
both ignored git-dir specified, and instead made $cwd as GIT_DIR.

This changes --bare not to override existing GIT_DIR.

This has been like this for a long time.  Let's hope nobody sane
relied on this insane behaviour.

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

Documentation/git.txt

@@ -120,7 +120,10 @@ OPTIONS
 	variable.
 
 --bare::
-	Same as --git-dir=`pwd`.
+	Treat the repository as a bare repository.  If GIT_DIR
+	environment is not set, it is set to the current working
+	directory.
+
 
 FURTHER DOCUMENTATION
 ---------------------

git.c

@@ -94,7 +94,7 @@ static int handle_options(const char*** argv, int* argc, int* envchanged)
 		} else if (!strcmp(cmd, "--bare")) {
 			static char git_dir[PATH_MAX+1];
 			is_bare_repository_cfg = 1;
-			setenv(GIT_DIR_ENVIRONMENT, getcwd(git_dir, sizeof(git_dir)), 1);
+			setenv(GIT_DIR_ENVIRONMENT, getcwd(git_dir, sizeof(git_dir)), 0);
 			if (envchanged)
 				*envchanged = 1;
 		} else {