Ignore git diff errors when there's no origin branch

[dbwiddis]

Description

The git diff command used for local code coverage reports fails when there is no origin to diff against. This ignores the return value which may occur on build environments.

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[codecov]

Codecov Report

Attention: 19 lines in your changes are missing coverage. Please review.

Comparison is base (56ccb1d) 68.39% compared to head (46d9cd4) 71.12%.
Report is 3 commits behind head on main.

Files	Patch %	Lines
...flowframework/workflow/RegisterLocalModelStep.java	91.13%	4 Missing and 3 partials ⚠️
...a/org/opensearch/flowframework/model/Template.java	92.15%	3 Missing and 1 partial ⚠️
...lowframework/workflow/RegisterRemoteModelStep.java	86.95%	1 Missing and 2 partials ⚠️
...ensearch/flowframework/workflow/GetMLTaskStep.java	92.85%	1 Missing and 1 partial ⚠️
...framework/indices/FlowFrameworkIndicesHandler.java	0.00%	1 Missing ⚠️
.../opensearch/flowframework/model/WorkflowState.java	0.00%	1 Missing ⚠️
...pensearch/flowframework/util/RestHandlerUtils.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #167      +/-   ##
============================================
+ Coverage     68.39%   71.12%   +2.73%     
- Complexity      339      368      +29     
============================================
  Files            45       47       +2     
  Lines          1585     1728     +143     
  Branches        143      154      +11     
============================================
+ Hits           1084     1229     +145     
+ Misses          452      443       -9     
- Partials         49       56       +7     

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[dbwiddis]

Closing in favor of #166