How can I visualize per-character differences in a unified diff file?

Here are some versions with less noisy output than git diff --word-diff-regex=<re> and that require less typing than, but are equivalent to, git diff --color-words --word-diff-regex=<re>.

Simple (does highlight space changes):

git diff --color-words

Simple (highlights individual character changes; does not highlight space changes):

git diff --color-words=.

More complex (does highlight space changes):

git diff --color-words='[^[:space:]]|([[:alnum:]]|UTF_8_GUARD)+'

In general:

git diff --color-words=<re>

where <re> is a regexp defining "words" for the purpose of identifying changes.

These are less noisy in that they color the changed "words", whereas using just --word-diff-regex=<re> surrounds matched "words" with colored -/+ markers.

In git, you can merge without committing. Merge your patch first, then do:

git diff --word-diff-regex=.

Note the dot after the equals sign.