Fix issue with hydrating nested fragments that are adjacent in the dom

[DylanPiercey]

Description

Currently in situations where there are nested fragments that are siblings it is possible that a fragment will find the incorrect ending marker which can cause strange behavior after hydrating.

This PR resolves the issue by keeping track of the number of open fragments discovered while looking for the end marker.

Checklist:

• I have read the CONTRIBUTING document and have signed (or will sign) the CLA.
• I have updated/added documentation affected by my changes.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Merging #1294 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1294      +/-   ##
==========================================
+ Coverage   90.53%   90.54%   +<.01%     
==========================================
  Files         339      339              
  Lines       12403    12411       +8     
==========================================
+ Hits        11229    11237       +8     
  Misses       1174     1174

Impacted Files	Coverage Δ
src/components/Component.js	95.28% <ø> (-0.02%)	⬇️
src/morphdom/index.js	98.04% <100%> (+0.07%)	⬆️

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[coveralls]

Coverage increased (+0.006%) to 90.541% when pulling f64de9a on fix-nested-adjacent-fragments into 856185d on master.