[gimp] devel-docs: add a tip for testing an older GIMP version with flatpak.

[Jehan <jehanp src gnome org>]

commit 6449dd7c141121b400ba672764ac561aed1563be
Author: Jehan <jehan girinstud io>
Date:   Mon Jun 1 17:21:29 2020 +0200

    devel-docs: add a tip for testing an older GIMP version with flatpak.
    
    Flatpak repositories can store successive builds (up to 20 in the
    Flathub repository in particular), even though by default it installs
    only the last build.
    I added in `debugging-tips.txt` the commands to search for and
    explicitly install an older build of our official GIMP build. This can
    be useful to compare some behaviorial changes as reported by users,
    without having to re-compile old commits just for a quick test.

 devel-docs/debugging-tips.txt | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)
---
diff --git a/devel-docs/debugging-tips.txt b/devel-docs/debugging-tips.txt
index 660661b78a..ae36a9b804 100644
--- a/devel-docs/debugging-tips.txt
+++ b/devel-docs/debugging-tips.txt
@@ -96,6 +96,28 @@ odd minor version) pop up a debug console at start.
 If you are building stable versions of GIMP for debugging and want this
 debug console as well, configure with `--enable-win32-debug-console`.
 
+## Testing older GIMP versions ##
+
+A useful trick when you want to quickly test a specific GIMP older
+version (e.g. to confirm a behavior change) is to install it with our
+official flatpak. The flathub repository stores past builds (up to 20 at
+the time of writing). You can list them with the following command:
+
+```
+$ flatpak remote-info --log flathub org.gimp.GIMP
+```
+
+Each build will have a "Subject" line (a comment to indicate the build
+reason, it may be just dependency updates or build fixes, or sometimes a
+bump in GIMP version) and a commit hash. When you have identified the
+build you want to test, update it like this:
+
+```
+flatpak update --commit=<hash-of-build> org.gimp.GIMP
+```
+
+Then just run your older GIMP!
+
 ## Debugging icons ##
 
 See file `devel-docs/icons.txt` to learn more about our icons, and in