jj: Add revset alias for current git head

Adds a very simple 'githead' revset alias. This is not much simpler than
directly invoking `git_head()` but it will remind me in the future of
its existence.

vcs/jj/config/jj/config.toml

@@ -40,3 +40,4 @@ log = "ancestors(@, 5) | ancestors(trunk()..(visible_heads() & mine()), 2) | tru
 "downstream(x,y)" = "(x::y) & y"
 "branches" = "downstream(trunk(), bookmarks()) & mine()"
 "curbranch" = "latest(branches::@- & branches)"
+"githead" = "::git_head()"