doc/Makefile: do not include hg.1.gendoc.txt in MANIFEST
It is an intermediate file used to produce the hg.1 and .hg.1.html
files and is not useful for people who download the tarball. It will
be regenerated automatically by the Makefile if users want to rebuild
the documentation.
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24