feat(scm): Add revset aliases for next and prev (#448)

Summary:
feat(scm): Add revset aliases for `next` and `prev`

 In the same way you can `sl go top`, it seems logical to also have
`sl go next`. However, instead it is `sl next`.  While I normally remember to
do `sl prev`, I sometimes accidentally do `sl go prev` out of habbit and get an
error.  Instead, let's simply add two revset aliases for `next` and `prev` to
make sl more intuitive.

The revset functions were added by D43383303.
This change adds convenient aliases so one can use `next`, `prev` without typing `()`:

  next
  prev
  previous

Number of steps can still be specified using the function form:

  next(2)
  prev(3)
  previous(4)

Closes #444

Pull Request resolved: #448

Test Plan:
Automated tested blocked on #447

Manually tested by running `sl go next` and `sl go prev` and verifying I ended
up in the right spot.

Reviewed By: muirdm

Differential Revision: D43329058

Pulled By: quark-zju

fbshipit-source-id: 6fa5c6e6e41971f8f15b784586176b089c5f0027

eden/scm/lib/config/loader/src/builtin_static/sapling.rs

@@ -426,8 +426,11 @@ style=sl_default
 [revsetalias]
 sb(n)=first(sort(bookmark(), -rev), n)
 sba=sort(bookmark(), -rev)
-top=heads(. ::)
-bottom=first(draft() & ::.)
+top=top()
+bottom=bottom()
+next=next()
+prev=previous
+previous=previous()
 base=last(public() & ::.)
 obsrelated(x)=mutrelated(x)
 focusedsmartlog(x)=focusedbranch(x) + draftbranch(x)^ + present(master)

eden/scm/tests/test-movement-revset.t

@@ -38,6 +38,21 @@ Linear stack:
   $ sl log -r 'bottom()'
   E
 
+With revset aliases:
+
+  $ sl config -q --local 'revsetalias.prev=previous' 'revsetalias.previous=previous()'
+
+  $ sl log -r prev
+  F
+  $ sl log -r 'prev(2)'
+  E
+  $ sl log -r 'previous'
+  F
+  $ sl log -r 'previous()'
+  F
+  $ sl log -r 'previous(2)'
+  E
+
 Multiple choices:
 
   $ newclientrepo