I praise etckeeper enough. That you use or not formal configuration management tools, you should have etckeeper installed, even if to do ad-hoc changes in an emergency.

For example, I just thought I could use it to figure out which Debian release I originally installed on this machine, and when I did the upgrades. Turns out it's amazingly easy:

cd /etc
sudo git log --oneline -p --no-prefix  /etc/issue

Here is the result here:

root@marcos:/etc# git log --oneline -p --no-prefix --pretty=format:'%s (%ai)' /etc/issue | cat
Initial commit (2011-02-23 00:14:08 -0500)
diff --git issue issue
new file mode 100644
index 0000000..9d52ed2
--- /dev/null
+++ issue
@@ -0,0 +1,2 @@
+Debian GNU/Linux 6.0 \n \l
+

saving uncommitted changes in /etc prior to apt run (2011-05-22 19:08:20 -0400)
diff --git issue issue
index 9d52ed2..647d490 100644
--- issue
+++ issue
@@ -1,2 +1,2 @@
-Debian GNU/Linux 6.0 \n \l
+Debian GNU/Linux wheezy/sid \n \l


committing changes in /etc after apt run (2013-01-23 09:40:27 -0500)
diff --git issue issue
index 647d490..d363ace 100644
--- issue
+++ issue
@@ -1,2 +1,2 @@
-Debian GNU/Linux wheezy/sid \n \l
+Debian GNU/Linux 7.0 \n \l


committing changes in /etc after apt run (2013-06-15 13:02:04 -0400)
diff --git issue issue
index d363ace..efc8255 100644
--- issue
+++ issue
@@ -1,2 +1,2 @@
-Debian GNU/Linux 7.0 \n \l
+Debian GNU/Linux 7 \n \l


committing changes in /etc after apt run (2014-02-02 23:28:12 -0500)
diff --git issue issue
index efc8255..e65d112 100644
--- issue
+++ issue
@@ -1,2 +1,2 @@
-Debian GNU/Linux 7 \n \l
+Debian GNU/Linux jessie/sid \n \l


committing changes in /etc after apt run (2014-12-18 11:32:43 -0500)
diff --git issue issue
index e65d112..6478eed 100644
--- issue
+++ issue
@@ -1,2 +1,2 @@
-Debian GNU/Linux jessie/sid \n \l
+Debian GNU/Linux 8 \n \l

In other words:

Unfortunately, there isn't much more details about the exact upgrade points, especially since /etc/os-release is a symlink starting with Jessie. Besides things are much more in flux than we would like to believe, especially when you run a rolling distribution like "testing", but it still gives a good idea of my upgrade history:

There is of course more information available directly in git log, namely the exact package version changes. With some more commandline filtering, we can see exactly when each upgrade was done, including minor releases:

# git log --date iso --grep base-files --reverse | egrep "^commit|^Date|base-files"
commit 34ad962ff10c6e4e201378698e0fe0d4b03c8c39
Date:   2011-04-02 22:21:11 -0400
    -base-files 6.0
    +base-files 6.0squeeze1
commit 1cba7d2e097091e86eba1a2d8e4f5a9771e746a1
Date:   2011-07-09 19:40:05 -0400
    -base-files 6.3
    +base-files 6.4
commit cb409a0fbe2f95c3cd6a7c1ff0af263b55c7d597
Date:   2011-09-28 17:54:57 -0400
    -base-files 6.4
    +base-files 6.5
commit cf6d9dab6f79f4b50e5bd80dcba1769b0aa6c84b
Date:   2012-03-25 19:18:44 -0400
    -base-files 6.5
    +base-files 6.7
commit bb1b6aab4406276388542cefd7b4eff92d960533
Date:   2012-06-29 00:42:32 -0400
    -base-files 6.7
    +base-files 6.9
commit c6d9218ba75b3276ea44d949ef3410c35713d487
Date:   2012-09-29 14:25:01 -0400
    -base-files 6.9
    +base-files 6.11
commit bd730398e572c8403b2b9c0421df64a407669877
Date:   2013-01-23 09:40:27 -0500
    -base-files 6.11
    +base-files 7.1
commit af42616d72c4fe5c4a0e43ee8948031732735ec5
Date:   2013-06-15 13:02:04 -0400
    -base-files 7.1
    +base-files 7.1wheezy1
commit d2ef5df9c689073b62b4898a12e42bb8488c8cdc
Date:   2013-10-14 18:15:11 -0400
    -base-files 7.1wheezy1
    +base-files 7.1wheezy2
commit 104ea49559eb2b6b2aff21817b3980b274882a28
Date:   2013-12-14 11:07:09 -0500
    -base-files 7.1wheezy2
    +base-files 7.1wheezy3
commit bfa19b7ad737cb22c495af9429e922e6ec46202d
Date:   2014-02-02 23:28:12 -0500
    -base-files 7.1wheezy3
    +base-files 7.2
commit 830570ca4083af0c4b87c6c7c746c9513e260cf7
Date:   2014-05-15 11:20:13 -0400
    -base-files 7.2
    +base-files 7.3
commit 4c9e6ca21bb8189810199e6d518250432a62391d
Date:   2014-07-27 19:50:14 -0400
    -base-files 7.3
    +base-files 7.5
commit 4bc6abe2ba193cb6fac4eae00fa855eb32b86400
Date:   2014-10-20 16:14:09 -0400
    -base-files 7.5
    +base-files 7.6
commit a5e2ce476982201338220a314b1f9ccb03c99517
Date:   2014-11-28 19:45:56 -0500
    -base-files 7.6
    +base-files 7.10
commit eaa69b112fee60ef71938d9a725c07f9f29b2011
Date:   2014-12-18 11:32:43 -0500
    -base-files 7.10
    +base-files 8
commit f95a199df77030dc9ee6ab55bf4fa246fa88c959
Date:   2015-07-24 12:34:04 -0400
    -base-files 8
    +base-files 8+deb8u1
commit db4ab55b7327cecea54c5fe6a65560ba0e385978
Date:   2015-09-07 19:09:47 -0400
    -base-files 8+deb8u1
    +base-files 8+deb8u2