git-branchless: install man pages

This fixes the failure of `git cmd --help` where `cmd` is some
git-branchless alias.

The error is due to a quirk of git: when supplied with the long form
`--help` flag, the git binary eats the flag and instead look for man
page of `cmd`. This errors out unless the man page for `cmd` is properly
installed. In the case of `git-branchless`, this can be achieved with
the command `git-branchless install-man-pages`, as implemented in this
patch.

pkgs/applications/version-management/git-branchless/default.nix

@@ -37,6 +37,10 @@ rustPlatform.buildRustPackage rec {
     libiconv
   ];
 
+  postInstall = ''
+    $out/bin/git-branchless install-man-pages $out/share/man
+  '';
+
   preCheck = ''
     export TEST_GIT=${git}/bin/git
     export TEST_GIT_EXEC_PATH=$(${git}/bin/git --exec-path)