tig --first-parent -m does not show diffs like git log -p --first-parent -m

[ivan]

I use git log -p --first-parent -m to look at a repository that makes heavy use of merges, but of course, I'd much rather be using tig. Unfortunately, tig doesn't show the diffs I expect, and I don't know if this is a bug or some unfortunate defaults that I have to work around.

For background, git log's -m is:

-m
  This flag makes the merge commits show the full diff like regular commits;
  for each merge parent, a separate log entry and diff is generated. An exception
  is that only diff against the first parent is shown when --first-parent option
  is given; in that case, the output represents the changes the merge brought
  into the then-current branch.

Here's a repro case:

git clone https://github.com/NixOS/nixpkgs
cd nixpkgs
git checkout 117680b0e9f4472e6161427ac0814fc2c6f998ba
tig --first-parent -m

and press Enter on the commit.

For NixOS/nixpkgs@117680b, tig --first-parent -m shows a ~27662 line diff, while git log -p --first-parent -m shows the ~45 line diff I expect.

# tig --version
tig version 2.4.0
ncursesw version 6.1.20180127
readline version 6.3

# git --version
git version 2.18.0

[ivan]

I figured out that git show shows the longer diffs when using git show -m instead of git show --first-parent -m, and that tig is indeed executing git show without --first-parent when showing the commit:

git show --encoding=UTF-8 --pretty=fuller --root --patch-with-stat --show-notes -m --no-color 117680b0e9f4472e6161427ac0814fc2c6f998ba --

I don't know why -m makes it into the arguments but --first-parent doesn't, but as a workaround, I found:

TIG_DIFF_OPTS=--first-parent tig --first-parent -m

[jamescassell]

Would it be possible to add a keyboard shortcut to enable this mode from within tig?