Move extra tag-fetching step into init script

This makes three related changes:

- Removes "git fetch --tags" from the instructions in the readme,
  because the goal of this command can be achieved in the
  init-tests-after-clone.sh script, and because this fetch command,
  as written, is probably only achieving that goal in narrow cases.
  In clones and fetches, tags on branches are fetched by default,
  and the tags needed by the tests are on branches. So the
  situations where "git fetch --tags" was helping were (a) when the
  remote recently gained the tags, and (b) when the original remote
  was cloned in an unusual way, not fetching all tags. In both
  cases, the "--tags" option is not what makes that fetch get the
  needed tags.

- Adds "git fetch --all --tags" to init-tests-after-clone.sh. The
  "--all" option causes it to fetch from all remotes, and this is
  more significant than "--tags", since the tags needed for testing
  are on fetched branches. This achieves what "git fetch --tags"
  was achieving, and it also has the benefit of getting tags from
  remotes that have been added but not fetched from, as happens
  with an upstream remote that was manually added with no further
  action. (It also gets branches from those remotes, but if master
  is on multiple remotes but at different commits then "git
  checkout master" may not be able to pick one. So do this *after*
  rather than before that.)

- Skips this extra fetch, and also the submodule cloning/updating
  step, when running on CI. CI jobs will already have taken care of
  cloning the repo with submodules recursively, and fetching all
  available tags. In forks without tags, the necessary tags for the
  test are not fetched--but the upstream remote is not set up on
  CI, so they wouldn't be obtained anyway, not even by refetching
  with "--all".

README.md

@@ -76,7 +76,6 @@ To clone the [the GitHub repository](https://github.com/gitpython-developers/Git
 ```bash
 git clone https://github.com/gitpython-developers/GitPython
 cd GitPython
-git fetch --tags
 ./init-tests-after-clone.sh
 ```
 
@@ -114,9 +113,9 @@ See [Issue #525](https://github.com/gitpython-developers/GitPython/issues/525).
 
 ### RUNNING TESTS
 
-_Important_: Right after cloning this repository, please be sure to have
-executed `git fetch --tags` followed by the `./init-tests-after-clone.sh`
-script in the repository root. Otherwise you will encounter test failures.
+_Important_: Right after cloning this repository, please be sure to have executed
+the `./init-tests-after-clone.sh` script in the repository root. Otherwise
+you will encounter test failures.
 
 On _Windows_, make sure you have `git-daemon` in your PATH. For MINGW-git, the `git-daemon.exe`
 exists in `Git\mingw64\libexec\git-core\`.

init-tests-after-clone.sh

@@ -19,4 +19,8 @@ git reset --hard HEAD~1
 git reset --hard HEAD~1
 git reset --hard HEAD~1
 git reset --hard __testing_point__
+
+test -z "$TRAVIS" || exit 0  # CI jobs will already have taken care of the rest.
+
+git fetch --all --tags
 git submodule update --init --recursive