[PATCH 1 of 2] commands: add "di" alias for "diff"

[Jun Wu]

Half a year ago, at fb, we shipped a pair of aliases "disableevolve",
"enableevolve". And we got a serious complaint soon after:

    hg: command 'di' is ambiguous:
    diff disableevolve
    Why?????? Can we kill this disableevolve thing? So many extra keystrokes
    plus retraining my brain

Therefore I'm very sensitive about this. I think we should always make sure
"d" = "diff" (although the complaint was only about "di").

It might be a good idea to add a "namepriority" option to commands, so
higher namepriority command wins a conflict without cluttering the help
text.

Excerpts from Gregory Szorc's message of 2016-11-05 11:26:11 -0700:
> # HG changeset patch
> # User Gregory Szorc <gregory.szorc at gmail.com>
> # Date 1478369272 25200
> #      Sat Nov 05 11:07:52 2016 -0700
> # Node ID 4bce42e7330311b58ecd38a4acbcd2d2dd1351ba
> # Parent  f01367faa792635ad2f7a6b175ae3252292b5121
> commands: add "di" alias for "diff"
> 
> In preparation of introducing `hg display`, we register "di" as an
> alias for "diff" so `hg di` isn't ambiguous. This will preserve
> backwards compatibility for anyone using `hg di` today.
> 
> diff --git a/mercurial/commands.py b/mercurial/commands.py
> --- a/mercurial/commands.py
> +++ b/mercurial/commands.py
> @@ -3786,7 +3786,7 @@ def debugwireargs(ui, repopath, *vals, *
>      if res1 != res2:
>          ui.warn("%s\n" % res2)
>  
> - at command('^diff',
> + at command('^diff|di',
>      [('r', 'rev', [], _('revision'), _('REV')),
>      ('c', 'change', '', _('change made by revision'), _('REV'))
>      ] + diffopts + diffopts2 + walkopts + subrepoopts,
> diff --git a/tests/test-help.t b/tests/test-help.t
> --- a/tests/test-help.t
> +++ b/tests/test-help.t
> @@ -296,7 +296,7 @@ Test short command list with verbose opt
>                   show changeset information by line for each file
>     clone         make a copy of an existing repository
>     commit, ci    commit the specified files or all outstanding changes
> -   diff          diff repository (or selected files)
> +   diff, di      diff repository (or selected files)
>     export        dump the header and diffs for one or more changesets
>     forget        forget the specified files on the next commit
>     init          create a new repository in the given directory
> @@ -504,6 +504,8 @@ Test command without options
>    $ hg help diff
>    hg diff [OPTION]... ([-c REV] | [-r REV1 [-r REV2]]) [FILE]...
>    
> +  aliases: di
> +  
>    diff repository (or selected files)
>    
>        Show differences between revisions for the specified files.