Improved prompt speed when hg is not installed.

Prevented bash from walking the path looking for hg when it is not
installed during each prompt refresh.

themes/base.theme.bash

@@ -34,7 +34,7 @@ function scm {
   if [[ -f .git/HEAD ]]; then SCM=$SCM_GIT
   elif [[ -n "$(git symbolic-ref HEAD 2> /dev/null)" ]]; then SCM=$SCM_GIT
   elif [[ -d .hg ]]; then SCM=$SCM_HG
-  elif [[ -n "$(hg root 2> /dev/null)" ]]; then SCM=$SCM_HG
+  elif which hg &> /dev/null && [[ -n "$(hg root 2> /dev/null)" ]]; then SCM=$SCM_HG
   elif [[ -d .svn ]]; then SCM=$SCM_SVN
   else SCM=$SCM_NONE
   fi