Smart-http: check if repository is OK to export before serving it

Similar to how git-daemon checks whether a repository is OK to be
exported, smart-http should also check.  This check can be satisfied
in two different ways: the environmental variable GIT_HTTP_EXPORT_ALL
may be set to export all repositories, or the individual repository
may have the file git-daemon-export-ok.

Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Tarmigan Casebolt <tarmigan+git@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-http-backend.txt

@@ -18,6 +18,11 @@ The program supports clients fetching using both the smart HTTP protcol
 and the backwards-compatible dumb HTTP protocol, as well as clients
 pushing using the smart HTTP protocol.
 
+It verifies that the directory has the magic file
+"git-daemon-export-ok", and it will refuse to export any git directory
+that hasn't explicitly been marked for export this way (unless the
+GIT_HTTP_EXPORT_ALL environmental variable is set).
+
 By default, only the `upload-pack` service is enabled, which serves
 'git-fetch-pack' and 'git-ls-remote' clients, which are invoked from
 'git-fetch', 'git-pull', and 'git-clone'.  If the client is authenticated,
@@ -70,6 +75,7 @@ Apache 2.x::
 +
 ----------------------------------------------------------------
 SetEnv GIT_PROJECT_ROOT /var/www/git
+SetEnv GIT_HTTP_EXPORT_ALL
 ScriptAlias /git/ /usr/libexec/git-core/git-http-backend/
 ----------------------------------------------------------------
 +
@@ -157,6 +163,10 @@ by the invoking web server, including:
 * QUERY_STRING
 * REQUEST_METHOD
 
+The GIT_HTTP_EXPORT_ALL environmental variable may be passed to
+'git-http-backend' to bypass the check for the "git-daemon-export-ok"
+file in each repository before allowing export of that repository.
+
 The backend process sets GIT_COMMITTER_NAME to '$REMOTE_USER' and
 GIT_COMMITTER_EMAIL to '$\{REMOTE_USER}@http.$\{REMOTE_ADDR\}',
 ensuring that any reflogs created by 'git-receive-pack' contain some