Fixed issue under gcov 8 where an un-hit section could overwrite a hit in templated code

[obiphi]

Hi,

When working with GCC 8 (ref #228) I found a bug whereby if the gcov file is arranged thus:

------------------
Foo<int>::Foo():
function Foo<int>::Foo() called 1 returned 100% blocks executed 100%
        1:    7:  Foo(): b (1000) {}
------------------
Foo<char>::Foo():
function Foo<char>::Foo() called 0 returned 0% blocks executed 0%
    #####:    7:  Foo(): b (1000) {}
------------------

instead of:

------------------
Foo<char>::Foo():
function Foo<char>::Foo() called 0 returned 0% blocks executed 0%
    #####:    7:  Foo(): b (1000) {}
------------------
Foo<int>::Foo():
function Foo<int>::Foo() called 1 returned 100% blocks executed 100%
        1:    7:  Foo(): b (1000) {}
------------------

then the line in question is flagged as un-hit, because the un-hit indicator #### occurs after the hit indicator. In addition, if there are multiple instantiations of the template with different types only the hit count for the last one is used.

I've modified an existing test to demonstrate & test the first issue, and validated that the change fixes the issue I see in my code as well. I have done my best to understand and follow the contributions rules and have run the build under Travis CI to verify the changes I've made.

Please let me know if there are any other changes I can make to this PR to get it accepted.

Thanks,
Phil

[codecov]

Codecov Report

Merging #315 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #315      +/-   ##
==========================================
+ Coverage   94.88%   94.89%   +<.01%     
==========================================
  Files          16       16              
  Lines        1878     1879       +1     
  Branches      328      328              
==========================================
+ Hits         1782     1783       +1     
  Misses         46       46              
  Partials       50       50

Impacted Files	Coverage Δ
gcovr/gcov.py	86.98% <100%> (ø)	⬆️
gcovr/tests/test_gcov_parser.py	100% <100%> (ø)	⬆️

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[latk]

This looks excellent! Thank you for the fix :)