fix(git): support single character branch names

In gitGraph, add support branch names that only have a single
character.

The branch regex is checking for a starting character, and an
ending character, so it currently needs at least two characters.

I've wrapped everything except the first character in a `()?` to fix
this.

There are some really complicated regexes that do match what
valid git branches are (see https://stackoverflow.com/a/12093994), but
I'm reluctant to add them in, since it will be a pain to test all
the different edgecases.

Hopefully https://github.com/mermaid-js/mermaid/pull/3432 might be used
in the future to make a better gitgraph parser!

src/diagrams/git/parser/gitGraph.jison

@@ -61,7 +61,7 @@ checkout(?=\s|$)                        return 'CHECKOUT';
 <string>["]                             this.popState();
 <string>[^"]*                           return 'STR';
 [0-9]+(?=\s|$)                          return 'NUM';
-\w[-\./\w]*[-\w]                        return 'ID'; // only a subset of https://git-scm.com/docs/git-check-ref-format
+\w([-\./\w]*[-\w])?                     return 'ID'; // only a subset of https://git-scm.com/docs/git-check-ref-format
 <<EOF>>                                 return 'EOF';
 \s+                                    /* skip all whitespace */ // lowest priority so we can use lookaheads in earlier regex