This fixes https://savannah.gnu.org/bugs/?51777

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Reported-by: David Lawrence Ramsey <pooka109@gmail.com>

Using do_up() and do_end() when the user types <Left> at the start of
a line, and do_down() and do_home() when typing <Right> at line's end
can be problematic when tabs are wider than the screen, because those
functions convert indexes to columns and back again twice, thus causing
inaccuracies.  Therefore, simply adjust current and current_x directly,
and then redraw the screen.

This fixes https://savannah.gnu.org/bugs/index.php?51778.

Regardless of whether the row start is at a multiple of the tab size.

This completes the fix for https://savannah.gnu.org/bugs/?51669.

This fixes https://savannah.gnu.org/bugs/?51770,
and fixes https://savannah.gnu.org/bugs/?51768

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Reported-by: David Lawrence Ramsey <pooka109@gmail.com>

This is a partial fix for https://savannah.gnu.org/bugs/?51770

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Analysis-by: David Lawrence Ramsey <pooka109@gmail.com>

Also rename a variable, to match the saving routine.

Also reshuffle an initialization and drop two useless asserts.

The preceding 'for' loop has already established that all the lines are
either unindentable or composed of only blanks.  So if now a line cannot
be unindented, it means it is fully blank, so it can be simply skipped.

This fixes https://savannah.gnu.org/bugs/?51760

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Reported-by: David Lawrence Ramsey <pooka109@gmail.com>

Also, reduce the scope of the 'line' variable.

Also, don't depend on statting the relative path, because if that would
fail, we would try to open a NULL pointer.

Also, trim or improve some comments.

Also, only unindent when all selected lines can be unindented,
which means that the relative indentation will be preserved.
For this purpose, it ignores empty lines and lines consisting
of only whitespace.

When unindenting is not possible, a message is shown.

This fixes https://savannah.gnu.org/bugs/?47684.

Instead of inserting the extra whitespace after the current indentation
of a line, add it to the start of the line.  This causes a fixed amount
of visual whitespace to be added regardless of whether --tabstospaces
is used or not.

This fixes http://savannah.gnu.org/bugs/?51438,
and its ancestor https://savannah.gnu.org/bugs/?51408.

This fixes https://savannah.gnu.org/bugs/?51669

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Reported-by: David Lawrence Ramsey <pooka109@gmail.com>

So that these functions have bindings by default, and easy bindings.
Add them to the help viewer too, so that searching backward becomes
possible there.

This also fixes https://savannah.gnu.org/bugs/?51735

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Reported-by: Urja Rannikko <urjaman@gmail.com>
Signed-off-by: Urja Rannikko <urjaman@gmail.com>
Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>

Tabs are of variable length depending on position, so allow them
to be displayed partially on one chunk and partially on another.

This fixes http://savannah.gnu.org/bugs/?51621.

Adding text plus whitespace while the cursor is at or near the topleft
corner of the edit window can cause a change in the preceding chunk,
throwing firstcolumn out of alignment.  Catch this special case.

This fixes http://savannah.gnu.org/bugs/?51743.

Further, slightly reword an error message so it is appropriate also
when an out-of-bounds file is specified on the command line.

There is no need to retain the (relative) path that the user
specified, so we can simply reuse that variable.