[WB-8055] Fix issue logging images with "/" on Windows

[KyleGoyette]

Fixes WB-8055

Description

Fixes a regression I introduced to allow users on windows to log files in different directories using "/"

Testing

Adds a test for this case

Checklist

• Name PR "[WB-NNNN][WB-MMMM] Add support for..." similar to entries in CHANGELOG.md
• Include reference to internal ticket "Fixes WB-NNNN" (and github issue "Fixes #NNNN" if applicable)

[codecov]

Codecov Report

Merging #3146 (02098e9) into master (06ced64) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3146      +/-   ##
==========================================
+ Coverage   80.00%   80.03%   +0.02%     
==========================================
  Files         213      213              
  Lines       28039    28039              
==========================================
+ Hits        22433    22440       +7     
+ Misses       5606     5599       -7     

Flag	Coverage Δ
functest	56.65% <100.00%> (ø)
unittest	69.44% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
wandb/util.py	85.58% <100.00%> (ø)
wandb/sdk/internal/sender.py	91.46% <0.00%> (-0.28%)	⬇️
wandb/sdk/lib/git.py	76.35% <0.00%> (ø)
wandb/sdk/wandb_run.py	88.75% <0.00%> (+0.27%)	⬆️
wandb/sdk/internal/meta.py	90.74% <0.00%> (+3.08%)	⬆️

[raubitsj]

4 character change, 4 new tests! perfect ratio