Update mandb after installing the man page

When manually installing the manpage, also update the mandb.

Signed-off-by: Eric Keller <keller.eric@gmail.com>

INSTALL.md

@@ -118,7 +118,7 @@ So, installation might look something like the following:
 
    ```
    cp -a git-filter-repo $(git --exec-path)
-   cp -a git-filter-repo.1 $(git --man-path)/man1
+   cp -a git-filter-repo.1 $(git --man-path)/man1 && mandb
    cp -a git-filter-repo.html $(git --html-path)
    ln -s $(git --exec-path)/git-filter-repo \
        $(python -c "import site; print(site.getsitepackages()[-1])")/git_filter_repo.py

Makefile

@@ -38,6 +38,7 @@ install: snag_docs #fixup_locale
 	ln -sf "$(bindir)/git-filter-repo" "$(pythondir)/git_filter_repo.py"
 	cp -a Documentation/man1/git-filter-repo.1 "$(mandir)/man1/git-filter-repo.1"
 	cp -a Documentation/html/git-filter-repo.html "$(htmldir)/git-filter-repo.html"
+	if which mandb > /dev/null; then mandb; fi
 
 
 #