gitfast: improve command aliases (#9385)

So that compdefs with dashes work as well as underscores:

  compdef _git gc=git_commit

Or:

  compdef _git gc=git-commit

The official Zsh Git completion uses dashes, and this way people don't
have to change their existing definitions.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>

plugins/gitfast/_git

@@ -275,6 +275,8 @@ _git ()
 		emulate ksh -c __${service}_main
 	elif (( $+functions[_${service}] )); then
 		emulate ksh -c _${service}
+	elif ((	$+functions[_${service//-/_}] )); then
+		emulate ksh -c _${service//-/_}
 	fi
 
 	let _ret && _default && _ret=0