[PATCH v2 0/4] builtin/diff: learn --merge-base

The range-notation in `git diff` has been cited as a mistake since diff compares two endpoints, not whole ranges.[0] In fact, the ranges seem to take on the opposite meanings when compared to range notation in `git log`.

In an effort to reduce the use of range notation as much as possible, introduce the `--merge-base` flag, slightly modified from a suggestion by Jonathan Nieder.[1] This flag allows us to replace the first commit given on the command-line with its merge base between the first and second commits. This allows us to gently deprecate the `...` form entirely, although that is left as an exercise to the reader ;)

One additional bonus is that this flag allows the "after" side to be not just constrained to a commit (like with `...` notation). It can now be the working tree or the index as well.

The `--merge-base` name isn't very satisfying. If anyone has any better suggestions, please let me know.

Changes since v1:

* Implement Junio's documentation suggestions

* Update git diff's usage to include this option

[0]: https://lore.kernel.org/git/xmqqy2v26hu0.fsf@gitster-ct.c.googlers.com/ [1]: https://lore.kernel.org/git/20191223215928.GB38316@google.com/

Denton Liu (4):
  t4068: remove unnecessary >tmp
  git-diff.txt: backtick quote command text
  builtin/diff: parse --no-index using parse_options()
  builtin/diff: learn --merge-base

 Documentation/git-diff.txt | 40 +++++++++++-----
 builtin/diff.c             | 94 +++++++++++++++++++++++++++++++++-----
 diff-no-index.c            | 15 ++----
 t/t4068-diff-symmetric.sh  | 89 ++++++++++++++++++++++++++++++++++--
 4 files changed, 198 insertions(+), 40 deletions(-)

Range-diff against v1:
1:  231ba3f661 ! 1:  4f219cf0d1 builtin/diff: learn --merge-base
    @@ Commit message
     
         Suggested-by: Jonathan Nieder <jrnieder@gmail.com>
     
    +
    + ## Notes ##
    +    The `--merge-base` name isn't very satisfying. If anyone has any
    +    suggestions for alternative names, please let me know.
    +
      ## Documentation/git-diff.txt ##
     @@ Documentation/git-diff.txt: SYNOPSIS
      'git diff' [<options>] [<commit>] [--] [<path>...]
    @@ Documentation/git-diff.txt: files on disk.
     +equivalent to `git diff $(git merge-base A B) B`.
     ++
     +In the case where one commit is given, a diff is displayed between the
    -+merge base and the working tree or the index if `--cached` is given.
    -+`git diff --merge-base A` is equivalent to `git diff $(git merge-base A
    -+HEAD)`.
    ++merge base of the commit and the HEAD and the working tree or the index
    ++if `--cached` is given. `git diff --merge-base A` is equivalent to `git
    ++diff $(git merge-base A HEAD)`.
     ++
     +In the case where no commits are given, this form behaves identically to
     +as if no `--merge-base` were supplied.
    @@ builtin/diff.c
      
      #define DIFF_NO_INDEX_EXPLICIT 1
      #define DIFF_NO_INDEX_IMPLICIT 2
    +@@ builtin/diff.c: static const char builtin_diff_usage[] =
    + "git diff [<options>] [<commit>] [--] [<path>...]\n"
    + "   or: git diff [<options>] --cached [<commit>] [--] [<path>...]\n"
    + "   or: git diff [<options>] <commit> [<commit>...] <commit> [--] [<path>...]\n"
    +-"   or: git diff [<options>] <commit>...<commit>] [--] [<path>...]\n"
    ++"   or: git diff [<options>] --merge-base [<commit> [<commit>]] [--] [<path>...]\n"
    + "   or: git diff [<options>] <blob> <blob>]\n"
    + "   or: git diff [<options>] --no-index [--] <path> <path>]\n"
    + COMMON_DIFF_OPTIONS_HELP;
     @@ builtin/diff.c: int cmd_diff(int argc, const char **argv, const char *prefix)
      	int blobs = 0, paths = 0;
      	struct object_array_entry *blob[2];
    @@ builtin/diff.c: int cmd_diff(int argc, const char **argv, const char *prefix)
      			   DIFF_NO_INDEX_EXPLICIT,
      			   PARSE_OPT_NONEG),
     +		OPT_BOOL(0, "merge-base", &merge_base,
    -+			 N_("use the merge base between the two commits as the diff base")),
    ++			 N_("compare with the merge base between two commits")),
      		OPT_END(),
      	};