fix #9284 (--- is incorrectly recognized as "the end of options")

[dungsaga]

No description provided.

[codecov]

Codecov Report

Merging #9285 (c25b125) into master (04b7b4b) will decrease coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #9285      +/-   ##
==========================================
- Coverage   90.19%   89.99%   -0.20%     
==========================================
  Files         151      151              
  Lines      170941   172719    +1778     
==========================================
+ Hits       154175   155440    +1265     
- Misses      16766    17279     +513     

Flag	Coverage Δ
huge-clang-none	89.32% <ø> (-0.80%)	⬇️
huge-gcc-none	89.76% <100.00%> (+<0.01%)	⬆️
huge-gcc-testgui	?
huge-gcc-unittests	2.45% <ø> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
src/xxd/xxd.c	86.47% <100.00%> (+0.88%)	⬆️
src/gui_gtk_x11.c	57.89% <0.00%> (-5.65%)	⬇️
src/gui_gtk.c	28.24% <0.00%> (-3.22%)	⬇️
src/highlight.c	87.41% <0.00%> (-2.97%)	⬇️
src/session.c	81.84% <0.00%> (-2.13%)	⬇️
src/buffer.c	90.42% <0.00%> (-1.96%)	⬇️
src/gui.c	69.57% <0.00%> (-1.72%)	⬇️
src/filepath.c	89.04% <0.00%> (-1.63%)	⬇️
src/hardcopy.c	86.15% <0.00%> (-1.62%)	⬇️
src/libvterm/src/vterm.c	66.06% <0.00%> (-1.38%)	⬇️
... and 99 more

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[brammool]

The xxd argument parsing is very basic, mostly only the first character is checked, thus instead of "-ps" you can also use "-ppp" or just "-p". Fixing one doesn't make much sense. Also, there should be a regression test.

[dungsaga]

@brammool I don't want to change the behavior of the argument parser in general. Its behavior is already documented. I just want to fix a small case when it's different from common convention.
I also added the tests for this fix.