shell: fix leaking strings

There are two memory leaks in "shell.c". The first one in `run_shell()`
is trivial and fixed without further explanation. The second one in
`cmd_main()` happens because we overwrite the `prog` variable, which
contains an allocated string. In fact though, the memory pointed to by
that variable is still in use because we use `split_cmdline()`, which
may create pointers into the middle of that string. But as we do not
have a direct pointer to the head of the allocated string anymore, we
get a complaint by the leak checker.

Address this by not overwriting the `prog` pointer.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9850-shell.sh

@@ -1,6 +1,8 @@
 #!/bin/sh
 
 test_description='git shell tests'
+
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 test_expect_success 'shell allows upload-pack' '