Add coalesce function to git-info

that will be used by the minimal theme, and could be used for future
themes too. Prezto had this, but we diminished its necessity by
simplifying how git-info works.

modules/git-info/functions/coalesce

@@ -0,0 +1,6 @@
+# Prints the first non-empty string in the arguments array.
+for arg in ${argv}; do
+  print -n ${arg}
+  return 0
+done
+return 1