BF: use get, not casting get_value while dealing with submodule path/url etc

[yarikoptic]

hopefully closes #613
did a quick shot, didn't do any tests besides locally:

(git)hopa:~deb/python-git[upstream-master]git
$> s=123; builtin cd /tmp/; rm -rf /tmp/gitsub; mkdir /tmp/gitsub; cd /tmp/gitsub; git init; mkdir $s; cd $s; git init; touch 1; git add 1; git commit -m msg 1; cd ..; git submodule add ./$s $s; git add .gitmodules; git commit -m "added subm $s"                                                                                                                                                                                                               Initialized empty Git repository in /tmp/gitsub/.git/                                        
Initialized empty Git repository in /tmp/gitsub/123/.git/
[master (root-commit) c883ce5] msg
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 1
123/
Adding existing repo at '123' to the index
[master (root-commit) 7c1e5c4] added subm 123
 2 files changed, 4 insertions(+)
 create mode 100644 .gitmodules
 create mode 160000 123
(dev) 2 10683.....................................:Thu 07 Sep 2017 11:21:59 PM EDT:.
(git)hopa:/tmp/gitsub[master]git
$> python -c 'import git; r=git.Repo("."); print(r.submodules)'
[git.Submodule(name=123, path=123, url=./123, branch_path=refs/heads/master)]
(dev) 2 10684.....................................:Thu 07 Sep 2017 11:22:00 PM EDT:.
(git)hopa:/tmp/gitsub[master]git
$> s=false; builtin cd /tmp/; rm -rf /tmp/gitsub; mkdir /tmp/gitsub; cd /tmp/gitsub; git init; mkdir $s; cd $s; git init; touch 1; git add 1; git commit -m msg 1; cd ..; git submodule add ./$s $s; git add .gitmodules; git commit -m "added subm $s"                                                                                                                                                                                                             
Initialized empty Git repository in /tmp/gitsub/.git/
Initialized empty Git repository in /tmp/gitsub/false/.git/
[master (root-commit) 02f47b7] msg
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 1
false/
Adding existing repo at 'false' to the index
[master (root-commit) a5547f5] added subm false
 2 files changed, 4 insertions(+)
 create mode 100644 .gitmodules
 create mode 160000 false
(dev) 2 10685.....................................:Thu 07 Sep 2017 11:22:06 PM EDT:.
(git)hopa:/tmp/gitsub[master]git
$> python -c 'import git; r=git.Repo("."); print(r.submodules)'
[git.Submodule(name=false, path=false, url=./false, branch_path=refs/heads/master)]

[yarikoptic]

ok -- ready to be merged! as #668 experiment showed -- last two runs of appveyor seems to be present also in current master, so have nothing to do with this PR.

[yarikoptic]

could even strip away us of str here... note that get is still advisable instead of get_value since it could cast+str 1.0 to 1 and then false to False.

[yarikoptic]

FWIW our battery of tests pass now, which puts more confidence into this fix, since we do use submodules extensively. quick merge/release would be appreciated

meanwhile, if there would be no activity, I will absorb the patch to at least (Neuro)Debian packages

[yarikoptic]

knock knock @Byron -- this PR seems to be quite an easy fix to digest, merge and it would be very appreciated... some folks might have had submodules like 20180911/ and now be out of luck ;)

[codecov-io]

Codecov Report

Merging #667 into master will increase coverage by 1.8%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #667     +/-   ##
=========================================
+ Coverage   92.57%   94.38%   +1.8%     
=========================================
  Files          61       61             
  Lines        9968     9969      +1     
=========================================
+ Hits         9228     9409    +181     
+ Misses        740      560    -180

Impacted Files	Coverage Δ
git/objects/submodule/base.py	94.54% <100%> (+2%)	⬆️
git/test/test_remote.py	97.82% <0%> (ø)	⬆️
git/refs/remote.py	91.11% <0%> (ø)	⬆️
git/compat.py	63.19% <0%> (+0.22%)	⬆️
git/refs/symbolic.py	96.5% <0%> (+0.31%)	⬆️
git/config.py	92.76% <0%> (+0.32%)	⬆️
git/test/test_submodule.py	99.22% <0%> (+0.38%)	⬆️
git/test/test_docs.py	100% <0%> (+0.39%)	⬆️
git/cmd.py	85.54% <0%> (+0.47%)	⬆️
git/repo/base.py	95.82% <0%> (+0.69%)	⬆️
... and 13 more

---

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