Decode base64-encoded text into a git_buf

When checking out files, we're performing conversion into the user's
native line endings, but we only want to do it for files which have
consistent line endings. Refuse to perform the conversion for mixed-EOL
files.

The CRLF->LF filter is left as-is, as that conversion is considered to be
normalization by git and should force a conversion of the line endings.

There are a few places where we need to join three strings to
assemble a path.  This adds a simple join3 function to avoid the
comparatively expensive join_n (which calls strlen on each string
twice).