doc: git-commit: apply new documentation guidelines

The documentation for git-commit has been updated to follow the new
documentation guidelines. The following changes have been applied to
the series of patches:

- switching the synopsis to a synopsis block which will automatically
  format placeholders in italics and keywords in monospace
- use _<placeholder>_ instead of <placeholder> in the description
- use `backticks for keywords and more complex option
descriptions`. The new rendering engine will apply synopsis rules to
these spans.

Additionally, some option descriptions have been turned into
imperative mood to make them more consistent with the rest of the
documentation.

Signed-off-by: Jean-Noël Avila <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jnavila