Do not attempt to return blob on submodule

[zeripath]

Fixes #6945 by not attempting to return a blob from a submodule.

[codecov-io]

Codecov Report

Merging #6996 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #6996      +/-   ##
=========================================
+ Coverage   41.49%   41.5%   +0.01%     
=========================================
  Files         440     440              
  Lines       59453   59453              
=========================================
+ Hits        24670   24676       +6     
+ Misses      31563   31558       -5     
+ Partials     3220    3219       -1

Impacted Files	Coverage Δ
modules/git/tree_blob.go	82.92% <100%> (ø)	⬆️
modules/log/event.go	65.98% <0%> (+1.52%)	⬆️
modules/log/file.go	77.62% <0%> (+2.09%)	⬆️

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[lunny]

@zeripath tests will be better.

[zeripath]

I really don't fancy trying to add tests for the compare endpoint. That would turn this one-line fix into a staggering behemoth, and I'd find and have to fix all of the rest of the bugs in compare.

If I start down that path I'll end up rewriting the whole damned diff architecture.