transports: ssh: clean up after libssh2 on exit

After calling `libssh2_init`, we need to clean up after the library by
executing `libssh2_exit` as soon as we exit. Register a shutdown handler
to do so which simply calls `libssh2_exit`. This fixes several memory
leaks.

src/global.c

@@ -22,7 +22,7 @@
 
 git_mutex git__mwindow_mutex;
 
-#define MAX_SHUTDOWN_CB 8
+#define MAX_SHUTDOWN_CB 9
 
 static git_global_shutdown_fn git__shutdown_callbacks[MAX_SHUTDOWN_CB];
 static git_atomic git__n_shutdown_callbacks;

src/transports/ssh.c

@@ -9,6 +9,7 @@
 #include <libssh2.h>
 #endif
 
+#include "global.h"
 #include "git2.h"
 #include "buffer.h"
 #include "netops.h"
@@ -893,6 +894,13 @@ int git_transport_ssh_with_paths(git_transport **out, git_remote *owner, void *p
 #endif
 }
 
+#ifdef GIT_SSH
+static void shutdown_ssh(void)
+{
+    libssh2_exit();
+}
+#endif
+
 int git_transport_ssh_global_init(void)
 {
 #ifdef GIT_SSH
@@ -901,6 +909,7 @@ int git_transport_ssh_global_init(void)
 		return -1;
 	}
 
+	git__on_shutdown(shutdown_ssh);
 	return 0;
 
 #else