ignoring changes matching a string in git diff

No more grep needed!

With Git 2.30 (Q1 2021), "git diff"^(man) family of commands learned the "-I<regex>" option to ignore hunks whose changed lines all match the given pattern.

See commit 296d4a9, commit ec7967c (20 Oct 2020) by Michał Kępień (kempniu).
^{(Merged by Junio C Hamano -- gitster -- in commit 1ae0949, 02 Nov 2020)}

diff: add -I<regex> that ignores matching changes

^{Signed-off-by: Michał Kępień}

Add a new diff option that enables ignoring changes whose all lines (changed, removed, and added) match a given regular expression.
This is similar to the -I/--ignore-matching-lines option in standalone diff utilities and can be used e.g. to ignore changes which only affect code comments or to look for unrelated changes in commits containing a large number of automatically applied modifications (e.g. a tree-wide string replacement).

The difference between -G/-S and the new -I option is that the latter filters output on a per-change basis.

Use the 'ignore' field of xdchange_t for marking a change as ignored or not.
Since the same field is used by --ignore-blank-lines, identical hunk emitting rules apply for --ignore-blank-lines and -I.
These two options can also be used together in the same git invocation (they are complementary to each other).

Rename xdl_mark_ignorable() to xdl_mark_ignorable_lines(), to indicate that it is logically a "sibling" of xdl_mark_ignorable_regex() rather than its "parent".

diff-options now includes in its man page:

-I<regex>

--ignore-matching-lines=<regex>

Ignore changes whose all lines match <regex>.
This option may be specified more than once.

Examples:

git diff --ignore-blank-lines -I"ten.*e" -I"^[124-9]"

---

A small memleak in "diff -I<regexp>" has been corrected with Git 2.31 (Q1 2021).

See commit c45dc9c, commit e900d49 (11 Feb 2021) by Ævar Arnfjörð Bjarmason (avar).
^{(Merged by Junio C Hamano -- gitster -- in commit 45df6c4, 22 Feb 2021)}

diff: plug memory leak from regcomp() on {log,diff} -I

^{Signed-off-by: Ævar Arnfjörð Bjarmason}

Fix a memory leak in 296d4a9 ("diff: add -I that ignores matching changes", 2020-10-20, Git v2.30.0-rc0 -- merge listed in batch #3) by freeing the memory it allocates in the newly introduced diff_free().

This memory leak was intentionally introduced in 296d4a9, see the discussion on a previous iteration of it.

At that time freeing the memory was somewhat tedious, but since it isn't anymore with the newly introduced diff_free() let's use it.

Let's retain the pattern for diff_free_file() and add a diff_free_ignore_regex(), even though (unlike "diff_free_file") we don't need to call it elsewhere.
I think this will make for more readable code than gradually accumulating a giant diff_free() function, sharing "int i" across unrelated code etc.

Try the following:

$ git diff > full_diff.txt
$ git diff -G "your pattern" > matching_diff.txt

You can then compare the two like so:

$ diff matching_diff.txt full_diff.txt

If all changes match the pattern, full_diff.txt and matching_diff.txt will be identical, and the last diff command will not return anything.

If there are changes that do not match the pattern, the last diff will highlight those.

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You can combine all of the above steps and avoid having to create two extra files like so:

diff <(git diff -G "your pattern") <(git diff)  # works with other diff tools too