improve git-diff documentation and A...B handling

git diff -h help is succinct, but perhaps too much so.

The symmetric-diff syntax, git diff A...B, is defined by the documentation to compare the merge base of A and B to commit B. It does so just fine when there is a merge base. It compares A and B directly if there is no merge base, and it is overly forgiving of bad arguments after which it can produce nonsensical diffs. It also behaves badly with other odd/incorrect usages, such as git diff A...B C..D.

The first patch simply adjusts a test that will fail if the second patch is accepted. The second patch adds special handling for the symmetric and range diff syntax so that the option parsing works, plus a small test suite. The third patch updates the documentation, including adding a section for combined commits, and makes the help output more verbose (to match the SYNOPSIS and provide common diff options like git-diff-files, for instance).

Changes since v3:

* correct > / >= goof
* fix test nit per Philip Oakley

Chris Torek (3):
t/t3430: avoid undefined git diff behavior
git diff: improve range handling
Documentation: usage for diff combined commits

 Documentation/git-diff.txt |  20 ++++--
 builtin/diff.c             | 132 +++++++++++++++++++++++++++++++++----
 t/t3430-rebase-merges.sh   |   2 +-
 t/t4068-diff-symmetric.sh  |  91 +++++++++++++++++++++++++
 4 files changed, 226 insertions(+), 19 deletions(-)
 create mode 100755 t/t4068-diff-symmetric.sh

base-commit: 20514004ddf1a3528de8933bc32f284e175e1012
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-804%2Fchris3torek%2Fcleanup-diff-v4
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-804/chris3torek/cleanup-diff-v4
Pull-Request: https://github.com/git/git/pull/804

Range-diff vs v3:

 1:  2ccaad645ff = 1:  2ccaad645ff t/t3430: avoid undefined git diff behavior
 2:  60aed3f9d65 ! 2:  4fa6fba33b3 git diff: improve range handling
     @@ builtin/diff.c: static int builtin_diff_files(struct rev_info *revs, int argc, c
      +			basecount++;
      +			break;		/* do mark all bases */
      +		case REV_CMD_LEFT:
     -+			if (lpos > 0)
     ++			if (lpos >= 0)
      +				usage(builtin_diff_usage);
      +			lpos = i;
      +			if (obj->flags & SYMMETRIC_LEFT) {
     @@ builtin/diff.c: static int builtin_diff_files(struct rev_info *revs, int argc, c
      +			}
      +			continue;
      +		case REV_CMD_RIGHT:
     -+			if (rpos > 0)
     ++			if (rpos >= 0)
      +				usage(builtin_diff_usage);
      +			rpos = i;
      +			continue;	/* don't mark B */
     @@ t/t4068-diff-symmetric.sh (new)
      +. ./test-lib.sh
      +
      +# build these situations:
     -+#  - normal merge with one merge base (b1...b2);
     -+#  - criss-cross merge ie 2 merge bases (b1...master);
     -+#  - disjoint subgraph (orphan branch, b3...master).
     ++#  - normal merge with one merge base (br1...b2r);
     ++#  - criss-cross merge ie 2 merge bases (br1...master);
     ++#  - disjoint subgraph (orphan branch, br3...master).
      +#
      +#     B---E   <-- master
      +#    / \ /

3: a7da92cd635 = 3: d7bc9aca44b Documentation: usage for diff combined commits

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