[PATCH v2 0/2] diff.c: fix a recent memory leak regression

There's a really dumb think-o in a commit of mine in the recently landed ccafbbfb4ee (Merge branch 'ab/plug-random-leaks', 2022-03-13), sorry about that, and thanks a lot to Michael J Gruber for the report.

This "v2" series is re-roll of his addition of a regression test in the "v1"[1].

As the range-diff shows I took the libertay of adjusting the commit message a bit, mainly noting the regression, re-wording a bit, and replacing the (presumably glibc?) output with the better SANITIZE=address report.

The 2/2 then fixes the issue, and changes the relevant regression tests to run under SANITIZE=leak (i.e. the "linux-leaks" job).

1. https://lore.kernel.org/git/a5e5cdd4658d457ffbd80f7263e352cbf3141a1a.1647520853.git.git@grubix.eu/

Michael J Gruber (1):
  tests: demonstrate "show --word-diff --color-moved" regression

Ævar Arnfjörð Bjarmason (1):
  diff.c: fix a double-free regression in a18d66cefb

 diff.c                     | 11 +++++++++--
 t/t4015-diff-whitespace.sh | 12 ++++++++++--
 2 files changed, 19 insertions(+), 4 deletions(-)

Range-diff against v1:
1:  f6e7318b418 ! 1:  7f6a6450259 tests: test show --word-diff --color-moved
    @@ Metadata
     Author: Michael J Gruber <git@grubix.eu>
     
      ## Commit message ##
    -    tests: test show --word-diff --color-moved
    +    tests: demonstrate "show --word-diff --color-moved" regression
     
    -    a18d66cefb ("diff.c: free "buf" in diff_words_flush()", 2022-03-04)
    -    introduced a breakage to `show --word-diff --color-moved` which gives
    +    Add a failing test which demonstrates a regression in
    +    a18d66cefb ("diff.c: free "buf" in diff_words_flush()", 2022-03-04),
    +    the regression is discussed in detail in the subsequent commit. With
    +    it running `git show --word-diff --color-moved` with SANITIZE=address
    +    would emit:
     
    -    free(): double free detected in tcache 2
    -    Aborted (core dumped)
    +            ==31191==ERROR: AddressSanitizer: attempting double-free on 0x617000021100 in thread T0:
    +                #0 0x49f0a2 in free (git+0x49f0a2)
    +                #1 0x9b0e4d in diff_words_flush diff.c:2153:3
    +                #2 0x9aed5d in fn_out_consume diff.c:2354:3
    +                #3 0xe092ab in consume_one xdiff-interface.c:43:9
    +                #4 0xe072eb in xdiff_outf xdiff-interface.c:76:10
    +                #5 0xec7014 in xdl_emit_diffrec xdiff/xutils.c:53:6
    +                [...]
     
    -    on every incarnation. This was not caught by the test suite because we
    -    test `diff --word-diff --color-moved` only so far.
    +            0x617000021100 is located 0 bytes inside of 768-byte region [0x617000021100,0x617000021400)
    +            freed by thread T0 here:
    +                #0 0x49f0a2 in free (git+0x49f0a2)
    +                [...(same stacktrace)...]
     
    -    Therefore, add a test for `show`, too.
    +            previously allocated by thread T0 here:
    +                #0 0x49f603 in __interceptor_realloc (git+0x49f603)
    +                #1 0xde4da4 in xrealloc wrapper.c:126:8
    +                #2 0x995dc5 in append_emitted_diff_symbol diff.c:794:2
    +                #3 0x96c44a in emit_diff_symbol diff.c:1527:3
    +                [...]
    +
    +    This was not caught by the test suite because we test `diff
    +    --word-diff --color-moved` only so far.
     
    -    Reverting a18d66cefb makes the test pass, but there might be a better
    -    fix.
    +    Therefore, add a test for `show`, too.
     
         Signed-off-by: Michael J Gruber <git@grubix.eu>
    +    Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
     
      ## t/t4015-diff-whitespace.sh ##
     @@ t/t4015-diff-whitespace.sh: test_expect_success 'cmd option assumes configured colored-moved' '
-:  ----------- > 2:  cae11491599 diff.c: fix a double-free regression in a18d66cefb