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.
Git-related fixes for themes.

Improved prompt display speed when git is not installed by
preventing bash from walking the path before each prompt.

Removed pesky stash-related git error message when in bare repos.
Merge pull request #275 from caguettaz/world

Improved prompt speed when hg is not installed.