Fix tree reporter line highlighting

[godlygeek]

Update tree reporter snapshots for Textual 0.49 (the TextArea no longer shows a cursor - yay!) and fix two bugs I noticed while looking at the snapshot diff: sometimes we don't show any lines, and sometimes we highlight the wrong line! Whoops.

[godlygeek]

My test program for making sure the line number handling is now right was:

import mmap


def a():
    b()
    b()


def b():
    c()


def c():
    e()


def e():
    f()


def f():
    g()


def g():
    h()


def h():
    i()


def i():
    j()


def j():
    mmap.mmap(-1, 1024 * 1024)


a()

That made it easy to test both the case where there's not enough lines above the active frame's line to fill out our context and the case where there are (and the case where the active frame's line is close to the end of the file rather than the start, too, but that doesn't need any special handling).

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (41248ed) 92.55% compared to head (fec7d8c) 92.82%.
Report is 6 commits behind head on main.

Files	Patch %	Lines
src/memray/reporters/tree.py	72.72%	3 Missing ⚠️
tests/conftest.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #544      +/-   ##
==========================================
+ Coverage   92.55%   92.82%   +0.27%     
==========================================
  Files          91       91              
  Lines       11304    11119     -185     
  Branches     1581     2022     +441     
==========================================
- Hits        10462    10321     -141     
+ Misses        837      798      -39     
+ Partials        5        0       -5     

Flag	Coverage Δ
cpp	92.82% <85.18%> (+6.88%)	⬆️
python_and_cython	92.82% <85.18%> (-2.90%)	⬇️

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