rn-13: add article about config --show-origin

Author: chriscool

rev_news/drafts/edition-13.md

@@ -21,9 +21,63 @@ This edition covers what happened during the month of February 2016.
 ### General
 -->
 
-<!---
 ### Reviews
--->
+
+*[config: add '--show-origin' option to print the origin of a config value](http://thread.gmane.org/gmane.comp.version-control.git/286198/)
+
+For some time Lars Schneider has been sending versions of a short
+patch series to make it possible to see where a config option comes
+from:
+
+-[v1](http://thread.gmane.org/gmane.comp.version-control.git/285553/)
+-[v2](http://thread.gmane.org/gmane.comp.version-control.git/285894/)
+-[v3](http://thread.gmane.org/gmane.comp.version-control.git/286110/)
+-[v4](http://thread.gmane.org/gmane.comp.version-control.git/286197/)
+-[v5](http://thread.gmane.org/gmane.comp.version-control.git/286485/)
+-[v6](http://thread.gmane.org/gmane.comp.version-control.git/286672/)
+
+Version 1 was itself based on a previous patch by Jeff King.
+
+The new feature could be useful because config options can be set in
+many ways. The usual way is to use one of the config files:
+
+- the ".git/config" file at the root of the working directory,
+- the user's "~/.gitconfig" file,
+- the user's "~/.config/git/config" file, or
+- a system wide "/etc/gitconfig" file.
+
+But the exact paths of the above files depend on how git was compiled
+and on the values of at least the GIT_CONFIG, GIT_CONFIG_NOSYSTEM,
+GIT_DIR and XDG_CONFIG_HOME environment variables.
+
+Also config values can also be set on the command line using the 'git
+-c' option, like `git -c <key>=<value> config ...`. Or they can be
+read from another file using the 'config -f' option, like `git config
+-f <file> ...`, or from the standard input. They can also be included
+from another file by using a "include.path = <path>" directive in a
+config file.
+
+As can be seen in the above threads, even if it seems simple, there
+are a lot of details to get right. These details include the name of
+the option. This was discussed by Sebastian Schuberth, Jeff King,
+Ramsay Jones, Mike Rappazzo and Junio Hamano. So the name was changed
+to '--show-origin'.
+
+Other details that were discussed are about the format of the output
+in special cases, like when the 'git -c' option is used, or when the
+'config -f' option is used, or when config is read from the standard
+input.
+
+The `git config` option also has a lot of different modes depending on
+which options it is passed, so it was discussed with which other
+options it is ok to pass '--show-origin'.
+
+Many details in the code and tests where also discussed by Eric
+Sunshine, Johannes Schindelin, Johannes Sixt, Jeff, Ramsay and Junio.
+
+One nice outcome of this patch series though is that error messages
+when there are problems in the config can now tell more precisely
+where the problems come from.
 
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 ### Support