2017-microprojects: convert $gmane references to links

We do not define the `$gmane` construct here at all. It's well known to list regulars, but probably not to applicants. Moreover, gmane is now defunct, so you have to know to dig up the link via public-inbox. Let's convert these to public-inbox references. And since this is real HTML, we can even make them hyperlinks. Amazing!

Author: peff

SoC-2017-Microprojects.md

@@ -144,6 +144,7 @@ This project requires a very good knowledge of regular expressions.
 ### Make "`git tag --contains <id>`" less chatty if `<id>` is invalid
 
 `git tag --contains <id>` prints the whole help text if `<id>` is invalid.
+It should only show the error message instead. [[thread](https://public-inbox.org/git/[email protected])]
 
 ### Git CI Improvements 1
 
@@ -208,14 +209,14 @@ for example.
 Pick one command that operates on branch names. Teach it the "-"
 shorthand that stands for "the branch we were previously on", like we
 did for "git merge -" sometime after we introduced "git checkout -".
-Cf. $gmane/230828
+[[thread](https://public-inbox.org/git/[email protected])]
 
 ### Use unsigned integral type for collection of bits.
 
 Pick one field of a structure that (1) is of signed integral type and (2) is
 used as a collection of multiple bits. Discuss if there is a good reason
 why it has to be a signed integral field and change it to an unsigned
-type otherwise. Cf. $gmane/263751
+type otherwise. [[thread](https://public-inbox.org/git/[email protected])]
 
 ### Move `~/.git-credential-cache` to `~/.cache/git`