Fix #749 - Use /bin/sh when the shell is fish

test/test_prepare_command.vader

@@ -0,0 +1,37 @@
+Before:
+  Save &shell
+  Save &shellcmdflag
+
+After:
+  Restore
+  let g:ale_has_override = {}
+
+Execute(sh should be used when the shell is fish):
+  " Set something else, so we will replace that too.
+  let &shellcmdflag = '-f'
+
+  let &shell = 'fish'
+
+  AssertEqual ['/bin/sh', '-c', 'foobar'], ale#job#PrepareCommand('foobar')
+
+  let &shell = '/usr/bin/fish'
+
+  AssertEqual ['/bin/sh', '-c', 'foobar'], ale#job#PrepareCommand('foobar')
+
+  let &shell = '/usr/local/bin/fish'
+
+  AssertEqual ['/bin/sh', '-c', 'foobar'], ale#job#PrepareCommand('foobar')
+
+Execute(Other shells should be used when set):
+  let &shell = '/bin/bash'
+  let &shellcmdflag = '-c'
+
+  AssertEqual ['/bin/bash', '-c', 'foobar'], ale#job#PrepareCommand('foobar')
+
+Execute(cmd /c as a string should be used on Windows):
+  let &shell = 'who cares'
+  let &shellcmdflag = 'whatever'
+
+  let g:ale_has_override = {'win32': 1}
+
+  AssertEqual 'cmd /c foobar', ale#job#PrepareCommand('foobar')