D3665: graph: improve graph output by using Unicode characters

[Yuya Nishihara]

>   A proper warning is now issued when the encoding is not UTF-8, or when East-Asian ambiguous characters will be rendered as wide characters.
>   The tests have been updated to check these warnings.
>   Yuya, I believe this should address everything we discussed. Let me know if you see a need for any further changes.

Yeah, the changes look good to me, thanks. Sean and others, can you review
and queue this if it seems good to go into the core extension? I'm 0 on this
feature.

A couple more nits:

```
--- tests/test-check-commit.t
+++ tests/test-check-commit.t.err
@@ -25,3 +25,14 @@
   >     fi
   >   done
   > fi
+  Revision 2c4617bda693 does not comply with rules
+  ------------------------------------------------------
+  167: adds double empty line
+   +
+  235: adds double empty line
+   +
+  1209: adds double empty line
+   +
+  1403: adds double empty line
+   +
+  

ERROR: test-check-commit.t output changed
!...
--- tests/test-check-pylint.t
+++ tests/test-check-pylint.t.err
@@ -19,3 +19,22 @@
   ------------------------------------ (?)
   Your code has been rated at 10.00/10 (?)
    (?)
+  Using config file $TESTTMP/fakerc
+  ************* Module hgext.beautifygraph
+  C: 38,29: More than one statement on a single line (multiple-statements)
+  C: 39,29: More than one statement on a single line (multiple-statements)
+  C: 40,29: More than one statement on a single line (multiple-statements)
+  C: 41,29: More than one statement on a single line (multiple-statements)
+  C: 42,29: More than one statement on a single line (multiple-statements)
+  C: 43,29: More than one statement on a single line (multiple-statements)
+  C: 44,29: More than one statement on a single line (multiple-statements)
+  C: 60,24: More than one statement on a single line (multiple-statements)
+  C: 61,24: More than one statement on a single line (multiple-statements)
+  C: 62,24: More than one statement on a single line (multiple-statements)
+  C: 63,24: More than one statement on a single line (multiple-statements)
+  C: 64,24: More than one statement on a single line (multiple-statements)
```

> # beautifygraph.py - improve graph output by using Unicode characters

Can you add a copyright line and GPL placeholder?

> A terminal with UTF-8 support and a monospace Unicode font are required.

Strictly speaking, the font doesn't matter. It's up to terminal emulator
how to lay out each character.

> from mercurial import encoding
> from mercurial import extensions
> from mercurial import templatekw
> from mercurial import graphmod

We prefer `from mercurial import (
    foo,
    bar,
    baz,
)`.