diff: support bare repositories when reading gitattributes for diff algorithm

This patch series adds support for bare repositories for the feature added in [1]. When using a bare repository, by default we will look for gitattributes from HEAD. When the --attr-source option is passed, we will try to read gitattributes from the commit.

A side effect of this patch series is that custom drivers will now also work with bare repositories.

1. (a4cf900ee7 diff: teach diff to read algorithm from diff driver,

    2022-02-20)

John Cai (2):
diff: use HEAD for attributes when using bare repository
diff: add --attr-source to read gitattributes from a commit

 Documentation/diff-options.txt  |  4 ++++
 Documentation/gitattributes.txt |  8 +++++++
 diff.c                          | 37 ++++++++++++++++++++++++++++++---
 diff.h                          |  1 +
 t/lib-diff-alternative.sh       | 33 ++++++++++++++++++++++++-----
 t/t4018-diff-funcname.sh        | 29 ++++++++++++++++++++++++++
 userdiff.c                      |  9 +++++++-
 userdiff.h                      |  4 ++++
 8 files changed, 116 insertions(+), 9 deletions(-)

base-commit: d15644fe0226af7ffc874572d968598564a230dd
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1459%2Fjohn-cai%2Fjc%2Fdiff-attr-bare-repo-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1459/john-cai/jc/diff-attr-bare-repo-v1
Pull-Request: https://github.com/git/git/pull/1459

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