Add manpage path to MANPATH for fish shell

This adds the manual page path for the git-subrepo man page when using
the fish startup script for git subrepo.

This is a not uncontentious issue it seems, in fish shell, see:
fish-shell/fish-shell#2090

This however seems to be the suggested solution, and appears to work
well, with the man page working along with system manpages, and
`manpage` command producing a reasonable result.

.fish.rc

@@ -4,7 +4,8 @@
 #
 # This is the `git-subrepo` initialization script.
 #
-# This script turns on the `git-subrepo` Git subcommand for the *Fish* shell.
+# This script turns on the `git-subrepo` Git subcommand and its manpages, for
+# the *Fish* shell.
 #
 # Just add a line like this to your `~/.config/fish/config.fish`:
 #
@@ -14,3 +15,6 @@
 
 set GIT_SUBREPO_ROOT (readlink -f (dirname (status --current-filename)))
 set PATH $GIT_SUBREPO_ROOT/lib $PATH
+
+set -q MANPATH || set MANPATH ''
+set MANPATH $MANPATH $GIT_SUBREPO_ROOT/man