gitignore: anchor all ignored names

gitignore: anchor all ignored names

by default, patterns/names in .gitignore are applied
recursively to all subdirectories.  So any name mentioned
in .gitignore is ignored in all subdirectores.  This is good
for, say. object files (*.o), but not good for particular
names which should be ignored only in one directory.  For
example, qemu-img.1 file is generated in the top directory,
and it should be ignored only there, not in some subdir.

At first, this might not matter much, but we have lots of
examples already where it actually does not help at all.
For example, top-level .gitignore ignores a file/dir named
"patches" (which is very questionable by itself), but it
is applied recursively, so git also ignores, for example,
debian/patches/ which should not be ignored.

So anchor all the names where appropriate.  .gitignore
should be cleaned up further, which will be addressed in
a subsequent patch.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>