Bug: combined diff with --ignore-matching-lines

["René Scharfe" <l.s.r@web.de>]

Hi all,

git diff segfaults when it's asked to produce a combined diff and ignore
certain lines with --ignore-matching-lines/-I, e.g.:

   $ git diff -I DEF_VER v2.33.3 v2.33.3^@
   zsh: segmentation fault  ./git-diff -I DEF_VER v2.33.3 v2.33.3^@

That's because combine-diff.c::diff_tree_combined() copies a diffopt
without making a deep copy of the ignore_regex array and frees it, then
later tries to use it.

The segfault can be fixed by adding "diffopt.no_free = 1;" or reverting
c45dc9cf30 (diff: plug memory leak from regcomp() on {log,diff} -I,
2021-02-11).

But even with that the only thing the command ignores is the option -I;
the GIT-VERSION-GEN changes in the middle should have been omitted:

   $ git diff -I DEF_VER v2.33.3 v2.33.3^@
   diff --cc Documentation/RelNotes/2.33.3.txt
   index 0000000000,0000000000..e2bada12a1
   new file mode 100644
   --- /dev/null
   +++ b/Documentation/RelNotes/2.33.3.txt
   @@@ -1,0 -1,0 +1,4 @@@
   ++Git Documentation/RelNotes/2.33.3.txt Release Notes
   ++=========================
   ++
   ++This release merges up the fixes that appear in v2.33.3.
   diff --cc GIT-VERSION-GEN
   index d81eab5f00,e7efe58866..86a3a2870c
   --- a/GIT-VERSION-GEN
   +++ b/GIT-VERSION-GEN
   @@@ -1,7 -1,7 +1,7 @@@
     #!/bin/sh

     GVF=GIT-VERSION-FILE
   - DEF_VER=v2.33.2
    -DEF_VER=v2.32.2
   ++DEF_VER=v2.33.3

     LF='
     '
   diff --cc RelNotes
   index 8e79de2efe,4ac68388c3..899139d9ec
   --- a/RelNotes
   +++ b/RelNotes
   @@@ -1,1 -1,1 +1,1 @@@
   - Documentation/RelNotes/2.33.2.txt
    -Documentation/RelNotes/2.32.2.txt
   ++Documentation/RelNotes/2.33.3.txt

Just setting need_generic_pathscan is not enough.  Ideas?

René