Support hgwhat alias

Support a Mercurial alias for listing the files that have changed on a
particular branch.

This commit is particularly noisy because I reformatted the above aliases to
align with the new width.

configs/shared/.config/fish/config.fish

@@ -236,6 +236,7 @@ abbr --add hgls  hg citc --list # should have different output from `pils`
 abbr --add hgrc   hg rebase --continue
 abbr --add hgra   hg rebase --abort
 abbr --add hgrm   hg citc -d # delete a CitC client created with Fig
+abbr --add hgwhat 'hg status --rev p4base:(hg id -i)'
 abbr --add hgconflicts hg resolve --list 'set:unresolved()' # much like `gconflicts`
 
 # Haskell