diff: deprecate double-dot diff syntax
To generate diffs between a set of commits, the git diff A B
syntax
can be used. There also exists an alternative double-dot syntax,
git diff A..B
which is functionally indentical. This double-dot
notation often gets confused with gitrevision(7) range notation. Since
the double-dot syntax is redundant and has nothing to do with revision
ranges, add a deprecation notice when it is used.
Signed-off-by: Justin Tobler jltobler@gmail.com