This will show a little not-existing sign in a buffer,
if that file lives in a git/hg repository but does not exists
there yet. Use `:let g:airline_symbols.notexists='!'` to configure
the '!' as symbol. By default, will use U+2204 symbol
To not impact performance by shelling out a lot, the result is cached
until the buffer is written or a shell command is issued.
Should work with mercurial and git.
fixes#925
d8adbfa135