not enough lines in _set_cache method

[ret2libc]

There is a list index out of range when printing the tag message of systemd repository

Reproducer:
clone systemd repository with `git clone https://github.com/systemd/systemd.git'

from git import Repo
r = Repo('./systemd')
t = r.tags['060']
if t.tag is not None:
    print(t.tag.message)

IndexError                                Traceback (most recent call last)
<ipython-input-15-462f7dbfed04> in <module>()
----> 1 t.tag.message

/usr/lib/python3.7/site-packages/gitdb/util.py in __getattr__(self, attr)
    254         to be created and set. Next time the same attribute is reqeusted, it is simply
    255         returned from our dict/slots. """
--> 256         self._set_cache_(attr)
    257         # will raise in case the cache was not created
    258         return object.__getattribute__(self, attr)

/usr/lib/python3.7/site-packages/git/objects/tag.py in _set_cache_(self, attr)
     60             self.tag = lines[2][4:]  # tag <tag name>
     61 
---> 62             tagger_info = lines[3]  # tagger <actor> <date>
     63             self.tagger, self.tagged_date, self.tagger_tz_offset = parse_actor_and_date(tagger_info)
     64 

IndexError: list index out of range

[Byron]

Thanks for this fantastic issue! I could easily reproduce it!
And I hope someone will have some time to dig deeper and figure out what this particular tag message is all about.

Here is the output of git cat-file which is what GitPython would see and parse:

➜  systemd git:(master) git cat-file tag 060
object 281c368b8f00fbe70ea45c3c5f8d152ae7f1e077
type commit
tag 060

And I think this shows the root of the issue - there simply is no tagger information, which is supposed to be on the next line.