Choppy scrolling in scm view with many files

[mjbvz]

Issue Type: Bug

repo

1. With a lot of files in the scm view
2. On a tall monitor so that many files are visible at a time
3. Scroll though the scm file list

Bug
Scrolling is noticeably choppy. When profiling, it looks like the creation of context keys is taking a lot of time for each scroll update

Here's my profile
Profile-20181206T173038.json.txt

VS Code version: code-oss-dev 1.30.0 (Commit unknown, Date unknown)
OS version: Darwin x64 18.2.0

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-4770HQ CPU @ 2.20GHz (8 x 2200)
GPU Status	2d_canvas: enabled checker_imaging: disabled_off flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: enabled multiple_raster_threads: enabled_on native_gpu_memory_buffers: enabled rasterization: enabled video_decode: enabled video_encode: enabled webgl: enabled webgl2: enabled
Load (avg)	3, 3, 3
Memory (System)	16.00GB (0.09GB free)
Process Argv	.
Screen Reader	no
VM	18%

[joaomoreno]

Will be fixed by #64503