[PATCH 11/12] builtin/rebase: fix options.strategy memory lifecycle

From: Andrzej Hunt <ajrhunt@google.com>

This change:
- xstrdup()'s all string being used for replace_opts.strategy, to
  guarantee that replace_opts owns these strings. This is needed because
  sequencer_remove_state() will free replace_opts.strategy, and it's
  usually called as part of the usage of replace_opts.
- Removes xstrdup()'s being used to populate options.strategy in
  cmd_rebase(), which avoids leaking options.strategy, even in the
  case where strategy is never moved/copied into replace_opts.

These changes are needed because:
- We would always create a new string for options.strategy if we either
  get a strategy via options (OPT_STRING(...strategy...), or via
  GIT_TEST_MERGE_ALGORITHM.
- But only sometimes is this string copied into replace_opts - in which
  case it did get free()'d in sequencer_remove_state().
- The rest of the time, the newly allocated string would remain unused,
  causing a leak. But we can't just add a free because that can result
  in a double-free in those cases where replace_opts was populated.

An alternative approach would be to set options.strategy to NULL when moving the pointer to replace_opts.strategy, combined with always free()'ing options.strategy, but that seems like a more complicated and wasteful approach.

This was first seen when running t0021 with LSAN, but t2012 helped catch the fact that we can't just free(options.strategy) at the end of cmd_rebase (as that can cause a double-free). LSAN output from t0021:

LSAN output from t0021:

Direct leak of 4 byte(s) in 1 object(s) allocated from:
    #0 0x486804 in strdup ../projects/compiler-rt/lib/asan/asan_interceptors.cpp:452:3
    #1 0xa71eb8 in xstrdup wrapper.c:29:14
    #2 0x61b1cc in cmd_rebase builtin/rebase.c:1779:22
    #3 0x4ce83e in run_builtin git.c:475:11
    #4 0x4ccafe in handle_builtin git.c:729:3
    #5 0x4cb01c in run_argv git.c:818:4
    #6 0x4cb01c in cmd_main git.c:949:19
    #7 0x6b3fad in main common-main.c:52:11
    #8 0x7f267b512349 in __libc_start_main (/lib64/libc.so.6+0x24349)

SUMMARY: AddressSanitizer: 4 byte(s) leaked in 1 allocation(s).

Signed-off-by: Andrzej Hunt <andrzej@ahunt.org>
---
 builtin/rebase.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)