[main-] remove unneeded reload in batch mode

[midichef]

Batch mode (when not replaying commands with -p) is substantially slower to load a file than interactive mode.

seq 2000111 > 2m.tsv
# interactive mode:  quit.vdj is a timing helper script that opens 2m.tsv and immediately quits
=time vd -p quit.vdj
6.41user
# batch mode
=time vd -b 2m.tsv
9.41user

quit.vdj.txt

That's because batch mode runs reload() twice in a row in these two lines:

visidata/visidata/main.py

Lines 336 to 337 in 581cdf1

	vd.push(sources[0])
	sources[0].reload()

vd.push() calls BaseSheet.ensureLoaded() which calls reload():

visidata/visidata/sheets.py

Line 1034 in 581cdf1

vs.ensureLoaded()

visidata/visidata/basesheet.py

Line 272 in 581cdf1

return self.reload() # likely launches new thread

This PR removes the unneeded explicit call to reload(), running 2x faster for the special case of no commands.

=time vd -b 2m.tsv
4.73user

[saulpw]

This line was added in 330117f. I agree that it shouldn't be necessary, and a 2x slowdown is bad of course, but let's make sure that we can still read from stdin in batch mode as per #354.

[midichef]

It looks like we can still read from stdin in batch mode:

# cat sample_data/UTF-8-demo.txt |vd -b -o test.batch.txt
saul.pw/VisiData v3.1dev
unknown "" filetype
Support VisiData: https://github.com/sponsors/saulpw
opening - as txt
saving 1 sheets to test.batch.txt as txt
# diff -s sample_data/UTF-8-demo.txt test.batch.txt
Files sample_data/UTF-8-demo.txt and test.batch.txt are identical

In addition, the specific sample command from #⁠354 also works for me: ls -l | vd -f fixed. So this PR is ready for final testing.