gitg crashes with assertion `g_slist_length (ce->priv->invalid) <= 1`

Hi,
I’m not sure whether this is really a new bug: there are a lot of bug report for gitg crashes, and it is difficult to investigate which is the same than mine.

There is a git repository on my computer in which gitg crashes. Here is its output:
```
(gitg:8104): GLib-GIO-CRITICAL **: 14:43:13.417: g_converter_convert: assertion 'outbuf_size > 0' failed

** (gitg:8104): WARNING **: 14:43:13.417: gitg-resource.vala:33: Error while loading resource: style-unix.css:1:0Failed to import: The resource at “/org/gnome/gitg/ui/style-unix.css” failed to decompress
**
GtkSourceView:ERROR:gtksourcecontextengine.c:5543:update_syntax: assertion failed: (g_slist_length (ce->priv->invalid) <= 1)
Aborted (core dumped)
```
Note that there are other git repositories on my computer in which gitg doesn’t crashes. Here is the output of gitg in another repository where it doesn’t crash:
```
(gitg:8913): GLib-GIO-CRITICAL **: 14:47:20.238: g_converter_convert: assertion 'outbuf_size > 0' failed

** (gitg:8913): WARNING **: 14:47:20.238: gitg-resource.vala:33: Error while loading resource: style-unix.css:1:0Failed to import: The resource at “/org/gnome/gitg/ui/style-unix.css” failed to decompress
```
So it seems that the first lines are normal.

The repository in which it fails can be found at https://github.com/Mbodin/murder-generator/tree/c5d43cc521d4002731a4c6f465a56324ba3732e6
When I launch gitg, its window appears very briefly, then it crashes without me doing anything. This is very quick: it all happens within a blink.

The bug is not deterministic: sometimes in this repository, the windows does appear. It is however wrong: in the commit list, only the three first commits appear (despite being much more than this in the current branch). When this happens, any interaction I have with the window just makes it crashes immediately, leaving the exact same error message, with the exact same assert error.

This scenario in which gitg doesn’t immediately crash is very rare: less than 1 out of 10 attemps.

Hoping it can help,
Regards,
Martin.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gitg 3.26.0-4
ProcVersionSignature: Ubuntu 4.15.0-1073.83-oem 4.15.18
Uname: Linux 4.15.0-1073-oem x86_64
ApportVersion: 2.20.9-0ubuntu7.11
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Sun Mar 15 14:43:24 2020
DistributionChannelDescriptor:
 # This is the distribution channel descriptor for the OEM CDs
 # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
 canonical-oem-somerville-bionic-amd64-20180608-47+berlinetta-cfl-r+X77
InstallationDate: Installed on 2019-08-28 (199 days ago)
InstallationMedia: Ubuntu 18.04 "Bionic" - Build amd64 LIVE Binary 20180608-09:38
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: gitg
UpgradeStatus: No upgrade log present (probably fresh install)