mercurial: add hga alias for "hg add" (#4969)

Add an alias for adding files in the mercurial plugin.

plugins/mercurial/mercurial.plugin.zsh

@@ -1,4 +1,5 @@
 # Mercurial
+alias hga='hg add'
 alias hgc='hg commit'
 alias hgb='hg branch'
 alias hgba='hg branches'