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