Add a simple git test

Just checks two completions and the prompt for now, but shows what is possible.

Work on #4249.

tests/checks/git.fish

@@ -0,0 +1,34 @@
+#RUN: %fish %s
+#REQUIRES: command -v git
+
+# Do some tests with `git` - completions are interesting,
+# but prompts would also be possible.
+
+set -l tmp (mktemp -d)
+
+cd $tmp
+git init >/dev/null 2>&1
+
+# Commands and descriptions
+# Note: We *can't* list all here because in addition to aliases,
+# git also uses all commands in $PATH called `git-something` as custom commands,
+# so this depends on system state!
+complete -C'git ' | grep '^add'
+# (note: actual tab character in the check here)
+#CHECK: add	Add file contents to the index
+
+touch foo
+
+complete -C'git add '
+#CHECK: foo	Untracked file
+
+# Note: We can't rely on the initial branch because that might be
+# "master", or it could be changed to something else in future!
+git checkout -b newbranch >/dev/null 2>&1
+fish_git_prompt
+echo # the git prompt doesn't print a newline
+#CHECK: (newbranch)
+
+__fish_git_prompt_show_informative_status=1 fish_git_prompt
+echo
+#CHECK: (newbranch %1)