Add alias for signed git commit with message (#5390)

Signed-off-by: Mat Munn <mat@savi.com.au>

plugins/git/git.plugin.zsh

@@ -65,6 +65,7 @@ alias gca!='git commit -v -a --amend'
 alias gcan!='git commit -v -a --no-edit --amend'
 alias gcans!='git commit -v -a -s --no-edit --amend'
 alias gcam='git commit -a -m'
+alias gcsm='git commit -s -m'
 alias gcb='git checkout -b'
 alias gcf='git config --list'
 alias gcl='git clone --recursive'