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
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 diff
syntax so that the option parsing works, plus a small test suite. The third
patch just updates the SYNOPSIS section of the documentation and makes the
help output more verbose (to match the SYNOPSIS and provide common diff
options like git-diff-files, for instance).