WIP: Update glom

[kurtbrose]

maybe this would be better covered by an improved Partial / Call

[codecov]

Codecov Report

Merging #106 into master will decrease coverage by 0.25%.
The diff coverage is 37.5%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
- Coverage   95.17%   94.92%   -0.26%     
==========================================
  Files          16       16              
  Lines        1824     1832       +8     
  Branches      270      270              
==========================================
+ Hits         1736     1739       +3     
- Misses         49       54       +5     
  Partials       39       39

Impacted Files	Coverage Δ
glom/mutation.py	93.75% <37.5%> (-5.12%)	⬇️

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[mahmoud]

In theory we could allow Merge() to update an existing dict.