Replace repo.namedBlob by git.TreeEntry. (#22898)

`namedBlob` turned out to be a poor imitation of a `TreeEntry`. Using
the latter directly shortens this code.

This partially undoes https://github.com/go-gitea/gitea/pull/23152/,
which I found a merge conflict with, and also expands the test it added
to cover the subtle README-in-a-subfolder case.

tests/gitea-repositories-meta/user2/repo1.git/objects/46/49299398e4d39a5c09eb4f534df6f1e1eb87cc

@@ -0,0 +1,4 @@
+x
<01><>QJ<51>0E<><45>*f><3E><><EFBFBD>I@D<><44>_<EFBFBD>!n`<60>L^<5E>m<EFBFBD>hS<1E><> <20><><EFBFBD><03>^<5E>
+e]<5D>
+<EFBFBD>3wu<13>n<18>zr<7A>,<2C><>]<5D>.6ԋ<36><D48B><EFBFBD>C<EFBFBD><43><0E>$u<>Mr<05><><EFBFBD><EFBFBD>
+1za<7A>I\<5C><><EFBFBD><EFBFBD><EFBFBD> 㘺<10>(><3E>T6x<36><78><17><1E>:<3A><0F><><EFBFBD><EFBFBD><EFBFBD>Oײ|<7C>u9~l"<22>i$c<10><> <20><>kZ[<5B><><1B>S<EFBFBD>

tests/gitea-repositories-meta/user2/repo1.git/refs/heads/sub-home-md-img-check

@@ -0,0 +1 @@
+4649299398e4d39a5c09eb4f534df6f1e1eb87cc