[git] add alias to get current branch name

This is useful for scripting

git/gitconfig.symlink

@@ -33,6 +33,8 @@
     # show what I did today
     day = "!sh -c 'git log --reverse --no-merges --branches=* --date=local --after=\"yesterday 11:59PM\" --author=\"`git config --get user.name`\"'"
     churn = "!f() { git log --all -M -C --name-only --format='format:' \"$@\" | sort | grep -v '^$' | uniq -c | sort | awk 'BEGIN {print \"count\tfile\"} {print $1 \"\t\" $2}' | sort -g; }; f"
+    # current branch
+    cbr = rev-parse --abbrev-ref HEAD
 
     # assume aliases
     assume = update-index --assume-unchanged