Tabs are never expanded, so tab-indented files render flush left

[fatbrother]

What happens

In a repository that indents with tabs, every line in the diff pane is drawn
starting at column 0.

Might like below

test: function(taskState)
{
aaaaa
vvvv

bbbb
if () 
{
}
},

Why

ratatui draws into a cell grid and does not interpret \t; the terminal never
sees the character either, so nothing expands it. gitview does not expand it
first — the only \t handling in the tree is field splitting:

src/git.rs:801              field.split('\t')       # git output columns
src/preview/session.rs:289  answer.split_once('\t') # pane picker reply

render::build hands old / new straight to Highlighter::highlight and
TextDiff::from_lines, so the raw tab survives into the Run text and is
dropped at draw time.

Reproduce

Just open any file with some indent with gitview.

Suggested fix

A width knob is worth having (tab_width, default 4 or 8) since it is per
project taste, and the expansion can be skipped entirely when neither side
contains a tab, so nothing is allocated for the common case.

I have a patch along these lines if it would be useful — it adds tab_width to
Config (clamped) and an expand_tabs helper applied at the top of build.